Remove redundant null check

Author: agilob

modules/openapi-generator/src/main/resources/dart2/api_client.mustache

@@ -72,7 +72,6 @@ class ApiClient {
     headerParams.addAll(_defaultHeaderMap);
 
     final urlEncodedQueryParams = queryParams
-      .where((param) => param.value != null)
       .map((param) => '$param');
 
     final queryString = urlEncodedQueryParams.isNotEmpty

samples/openapi3/client/petstore/dart2/petstore_client_lib/lib/api_client.dart

@@ -66,7 +66,6 @@ class ApiClient {
     headerParams.addAll(_defaultHeaderMap);
 
     final urlEncodedQueryParams = queryParams
-      .where((param) => param.value != null)
       .map((param) => '$param');
 
     final queryString = urlEncodedQueryParams.isNotEmpty

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/api_client.dart

@@ -69,7 +69,6 @@ class ApiClient {
     headerParams.addAll(_defaultHeaderMap);
 
     final urlEncodedQueryParams = queryParams
-      .where((param) => param.value != null)
       .map((param) => '$param');
 
     final queryString = urlEncodedQueryParams.isNotEmpty