refactor(easydocx): Objects.requireNonNull boundary checks (java-conventions)

- EasyDocx.write/read: model and listener rejected when null
- DocxWriterBuilder.process(Map): null vars handled as empty (was NPE
  in new HashMap<>(null))
- DocxReaderBuilder ctor: model/listener requireNonNull
- MarkdownConverter/DocxFields null->default already compliant
Tests: EasyDocxTest 5 + DocxWriterBuilderTest 4 + ReaderBuilder 3 (11 green).

Author: loong10k

easydoc-core/src/main/java/io/github/easy4j/doc/easy/DocxReaderBuilder.java

@@ -2,6 +2,7 @@
 
 import java.io.File;
 import java.util.HashMap;
+import java.util.Objects;
 import java.util.Map;
 
 import org.docx4j.openpackaging.packages.WordprocessingMLPackage;
@@ -24,8 +25,8 @@ public final class DocxReaderBuilder<T> {
 
 	public DocxReaderBuilder(File templateFile, Class<T> model, DocxReadListener<T> listener) {
 		this.templateFile = templateFile;
-		this.model = model;
-		this.listener = listener;
+		this.model = Objects.requireNonNull(model, "model must not be null");
+		this.listener = Objects.requireNonNull(listener, "listener must not be null");
 	}
 
 	/**

easydoc-core/src/main/java/io/github/easy4j/doc/easy/DocxWriterBuilder.java

@@ -2,6 +2,7 @@
 
 import java.io.File;
 import java.util.HashMap;
+import java.util.Objects;
 import java.util.Map;
 
 import org.docx4j.openpackaging.packages.WordprocessingMLPackage;
@@ -47,10 +48,12 @@ public WordprocessingMLPackage process(T data) throws Exception {
 		return process(DocxFields.from(data));
 	}
 
-	/** 原始 Map 渲染（兼容现有 API 的变量注入）。 */
+	/** 原始 Map 渲染（兼容现有 API 的变量注入）；vars 为 null 时按空变量处理。 */
 	public WordprocessingMLPackage process(Map<String, Object> vars) throws Exception {
 		WordprocessingMLTemplate template = DocxTemplates.create(mode);
-		Map<String, Object> effective = new HashMap<String, Object>(vars);
+		Map<String, Object> effective = vars != null
+				? new HashMap<String, Object>(vars)
+				: new HashMap<String, Object>();
 		return template.process(templateFile, effective);
 	}
 }

easydoc-core/src/main/java/io/github/easy4j/doc/easy/EasyDocx.java

@@ -1,6 +1,7 @@
 package io.github.easy4j.doc.easy;
 
 import java.io.File;
+import java.util.Objects;
 
 /**
  * 类 EasyExcel / easyodf 的 easydoc 静态门面：链式构建 docx 模板渲染与读取。
@@ -13,20 +14,26 @@ private EasyDocx() {
 	}
 
 	public static <T> DocxWriterBuilder<T> write(String templatePath, Class<T> model) {
+		Objects.requireNonNull(model, "model must not be null");
 		return new DocxWriterBuilder<T>(new File(templatePath), model);
 	}
 
 	public static <T> DocxWriterBuilder<T> write(File templateFile, Class<T> model) {
+		Objects.requireNonNull(model, "model must not be null");
 		return new DocxWriterBuilder<T>(templateFile, model);
 	}
 
 	public static <T> DocxReaderBuilder<T> read(String templatePath, Class<T> model,
 			DocxReadListener<T> listener) {
+		Objects.requireNonNull(model, "model must not be null");
+		Objects.requireNonNull(listener, "listener must not be null");
 		return new DocxReaderBuilder<T>(new File(templatePath), model, listener);
 	}
 
 	public static <T> DocxReaderBuilder<T> read(File templateFile, Class<T> model,
 			DocxReadListener<T> listener) {
+		Objects.requireNonNull(model, "model must not be null");
+		Objects.requireNonNull(listener, "listener must not be null");
 		return new DocxReaderBuilder<T>(templateFile, model, listener);
 	}
 }

easydoc-core/src/test/java/io/github/easy4j/doc/easy/DocxWriterBuilderTest.java

@@ -54,6 +54,15 @@ void processAcceptsRawMap() throws Exception {
 		tmp.delete();
 	}
 
+	@Test
+	void processHandlesNullVarsAsEmpty() throws Exception {
+		File tmp = tempTemplate();
+		WordprocessingMLPackage pkg = new DocxWriterBuilder<Contract>(tmp, Contract.class)
+				.process((java.util.Map<String, Object>) null);
+		assertNotNull(pkg, "null vars must render as empty variables");
+		tmp.delete();
+	}
+
 	@Test
 	void documentAndModeAreChainable() throws Exception {
 		File tmp = tempTemplate();

easydoc-core/src/test/java/io/github/easy4j/doc/easy/EasyDocxTest.java

@@ -1,6 +1,7 @@
 package io.github.easy4j.doc.easy;
 
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.io.File;
 
@@ -19,6 +20,25 @@ void writeReturnsWriterBuilder() {
 				"write(file, model) must return a builder");
 	}
 
+	@Test
+	void writeRejectsNullModel() {
+		assertThrows(NullPointerException.class,
+				() -> EasyDocx.write("src/test/resources/tpl/template.docx", null),
+				"null model must be rejected");
+	}
+
+	@Test
+	void readRejectsNullModelOrListener() {
+		DocxReadListener<Model> l = (data, values) -> {
+		};
+		assertThrows(NullPointerException.class,
+				() -> EasyDocx.read("src/test/resources/tpl/template.docx", null, l),
+				"null model must be rejected");
+		assertThrows(NullPointerException.class,
+				() -> EasyDocx.read("src/test/resources/tpl/template.docx", Model.class, null),
+				"null listener must be rejected");
+	}
+
 	@Test
 	void readReturnsReaderBuilder() {
 		DocxReadListener<Model> l = new DocxReadListener<Model>() {