Revert "Remove deprecated falsy handling of filter_page_image"

This reverts commit c9389c7713.

src/ocrmypdf/_sync.py

@@ -206,8 +206,8 @@ def exec_page_sync(page_context: PageContext) -> PageResult:
         filtered_image = page_context.plugin_manager.hook.filter_page_image(
             page=page_context, image_filename=visible_image_out
         )
-
-        visible_image_out = filtered_image
+        if filtered_image:
+            visible_image_out = filtered_image
         pdf_page_from_image_out = create_pdf_page_from_image(
             visible_image_out, page_context, orientation_correction
         )

src/ocrmypdf/pluginspec.py

@@ -276,19 +276,17 @@ def filter_page_image(page: PageContext, image_filename: Path) -> Path:
 
     If the return value is a file that does not exist, ``FileNotFoundError``
     will occur. The return value should be a path to a file in the same folder
-    as ``image_filename``. To leave the image unmodified, ``image_filename``
-    should be returned.
+    as ``image_filename``.
+
+    Implementation detail: If the value returned is falsy, OCRmyPDF will ignore
+    the return value and assume the input file was unmodified. This is deprecated.
+    To leave the image unmodified, ``image_filename`` should be returned.
 
     Note:
         This hook will be called from child processes. Modifying global state
         will not affect the main process or other child processes.
     Note:
         This is a :ref:`firstresult hook<firstresult>`.
-
-    .. versionchanged:: 14.0
-        Previously, OCRmyPDF would treat as a falsy value as a request to leave
-        the image unmodified. This is no longer supported and will trigger an
-        exception.
     """