fix: Don't pass an empty doc rev into ietf.doc.views_doc.document_bibtex

[larseggert]

Fixes #4878

[codecov]

Codecov Report

Merging #4880 (9d201db) into main (a7c0cc0) will increase coverage by 0.00%.
The diff coverage is 87.23%.

@@           Coverage Diff           @@
##             main    #4880   +/-   ##
=======================================
  Coverage   88.46%   88.46%           
=======================================
  Files         296      296           
  Lines       39755    39796   +41     
=======================================
+ Hits        35168    35205   +37     
- Misses       4587     4591    +4     

Impacted Files	Coverage Δ
ietf/api/views.py	89.47% <ø> (ø)
ietf/ietfauth/urls.py	100.00% <ø> (ø)
ietf/stats/views.py	93.10% <0.00%> (ø)
ietf/submit/forms.py	82.74% <0.00%> (ø)
ietf/utils/decorators.py	87.65% <0.00%> (ø)
ietf/utils/mail.py	79.62% <0.00%> (ø)
ietf/doc/models.py	88.16% <20.00%> (-0.29%)	⬇️
ietf/nomcom/forms.py	95.39% <50.00%> (ø)
ietf/mailinglists/models.py	89.65% <66.66%> (ø)
ietf/doc/forms.py	95.50% <93.75%> (-0.50%)	⬇️
... and 15 more

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[rjsparks]

This isn't the right fix.
The bug is at the top of the function - there's no way calling this with an fyi (aka an rfc) should have landed on that line of code.
I'll propose a different fix later today.

[rjsparks]

OBE by #4887