Fix internal server error when generating bibtex for legacy RFCs. But at least

for RFC1786, the authors field is empty and the date wrong. Need to check
whether this is an error in the database or in the code. But still: Commit ready
for merge.
 - Legacy-Id: 10676

Author: larseggert

ietf/doc/views_doc.py

@@ -675,7 +675,7 @@ def document_bibtex(request, name, rev=None):
     latest_revision = doc.latest_event(NewRevisionDocEvent, type="new_revision")
     replaced_by = [d.name for d in doc.related_that("replaces")]
     published = doc.latest_event(type="published_rfc")
-    rfc = latest_revision.doc if latest_revision.doc.get_state_slug() == "rfc" else None
+    rfc = latest_revision.doc if latest_revision and latest_revision.doc.get_state_slug() == "rfc" else None
 
     if rev != None and rev != doc.rev:
         # find the entry in the history

ietf/templates/doc/document_bibtex.bib

@@ -6,7 +6,7 @@
 @misc{% templatetag openbrace %}rfc{{ doc.rfc_number }},
 	series =	{Request for Comments},
 	number=		{{ doc.rfc_number }},
-	howpublished=	{% templatetag openbrace %}{{ doc.stream }} RFC {{ doc.rfc_number }} ({{ doc.intended_std_level }}){% templatetag closebrace %},
+	howpublished=	{% templatetag openbrace %}{{ doc.stream }} RFC {{ doc.rfc_number }}{% if doc.doc.intended_std_level %} ({{ doc.intended_std_level }}){% endif %}{% templatetag closebrace %},
 	publisher =	{RFC Editor},
 	doi =		{% templatetag openbrace %}10.17487/rfc{{ doc.rfc_number }}{% templatetag closebrace %},
 	url =		{https://rfc-editor.org/rfc/rfc{{ doc.rfc_number }}.txt},{% else %}