FIX: Gracefully handle empty files for imgmath depth extraction

Closes sphinx-doc#14466.

While empty files are likely an error `read_svg_depth()` is not the place to flag empty files. It is designed to handle files without depth comment, and that should also be true for the edge case of an empty file.

Author: timhoffm

sphinx/ext/imgmath.py

@@ -67,15 +67,17 @@ class InvokeError(SphinxError):
 
 
 def read_svg_depth(filename: str | os.PathLike[str]) -> int | None:
-    """Read the depth from comment at last line of SVG file"""
+    """Read the depth from comment at last line of SVG file.
+
+    Returns None if the file does not contain a depth comment.
+    """
     with open(filename, encoding='utf-8') as f:
-        for line in f:  # NoQA: B007
-            pass
-        # Only last line is checked
-        matched = depthsvgcomment_re.match(line)
-        if matched:
+        last_line = next(reversed(list(f)), None)
+
+    if last_line is not None:
+        if matched := depthsvgcomment_re.match(last_line):
             return int(matched.group(1))
-        return None
+    return None
 
 
 def write_svg_depth(filename: Path, depth: int) -> None: