fix: handle both rev is None and '' in document_main (ietf-tools#4887)

Author: jennifer-richards

ietf/doc/views_doc.py

@@ -162,7 +162,7 @@ def document_main(request, name, rev=None, document_html=False):
     snapshot = False
 
     gh = None
-    if rev != None:
+    if rev:
         # find the entry in the history
         for h in doc.history_set.order_by("-time"):
             if rev == h.rev: