OCR, Parsing: Revert iterator stop

Revert how the OCR and document parsing generators end
their iteration. Originally they issue an empty return,
then a blank yield was added. This commit reverts the
blank yield and restores the original 'return' behavior.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_parsing/utils.py

@@ -14,7 +14,7 @@ def get_document_content(document):
         try:
             page_content = page.content.content
         except DocumentPageContent.DoesNotExist:
-            yield ''
+            return
         else:
             yield conditional_escape(force_text(page_content))