fix: handle missing LibreOffice in DocConverter

Update DocConverter to return an error message when LibreOffice 
is not found. Add a new test to verify the error handling 
behavior when the `shutil.which` function is mocked to return 
None, ensuring that users receive clear instructions to 
install LibreOffice.
refactor(tests): update test_markitdown_soffice_mocked

src/markitdown/_markitdown.py

@@ -725,7 +725,10 @@ def convert(self, local_path, **kwargs) -> Union[None, DocumentConverterResult]:
             return None
 
         if not (soffice := shutil.which("soffice")):
-            return None
+            return DocumentConverterResult(
+                title=None,
+                text_content="[ERROR] LibreOffice not found. Please install LibreOffice and try again.",
+            )
 
         local_path = Path(local_path)
         outdir = local_path.parent

tests/test_markitdown.py

@@ -282,6 +282,14 @@ def test_markitdown_soffice() -> None:
         text_content = result.text_content.replace("\\", "")
         assert test_string in text_content
 
+def test_markitdown_soffice_mocked(mocker) -> None:
+    mocker.patch("shutil.which", return_value=None)
+    markitdown = MarkItDown()
+
+    # Test DOC processing
+    result = markitdown.convert(os.path.join(TEST_FILES_DIR, "test.doc"))
+    text_content = result.text_content.replace("\\", "")
+    assert text_content == "[ERROR] LibreOffice not found. Please install LibreOffice and try again."
 
 
 def test_markitdown_deprecation() -> None: