Fix skip condition

Author: SilverRainZ

sphinxnotes/snippet/picker.py

@@ -26,7 +26,7 @@ def pick(doctree:nodes.document) -> List[Tuple[Snippet,nodes.section]]:
     Pick snippets from document, return a list of snippet and the section
     it belongs to.
     """
-    if doctree.source is None or doctree.source == "":
+    if doctree.source == "":
         logger.debug('Skipped document with empty source')
         return []