fix: treat str input as None in the person_link tag

[jennifer-richards]

Fixes #3830

[codecov]

Codecov Report

Merging #3832 (ab54dbc) into main (ef4ad54) will increase coverage by 0.03%.
The diff coverage is 98.52%.

@@            Coverage Diff             @@
##             main    #3832      +/-   ##
==========================================
+ Coverage   87.95%   87.99%   +0.03%     
==========================================
  Files         296      297       +1     
  Lines       38795    38752      -43     
==========================================
- Hits        34124    34098      -26     
+ Misses       4671     4654      -17     

Impacted Files	Coverage Δ
ietf/meeting/models.py	86.08% <ø> (ø)
ietf/meeting/urls.py	100.00% <ø> (ø)
ietf/meeting/utils.py	90.27% <ø> (-0.29%)	⬇️
ietf/name/models.py	100.00% <ø> (ø)
ietf/meeting/views.py	91.02% <96.00%> (+0.42%)	⬆️
ietf/doc/templatetags/ballot_icon.py	84.89% <100.00%> (ø)
ietf/doc/templatetags/ietf_filters.py	80.81% <100.00%> (-0.44%)	⬇️
ietf/doc/views_ballot.py	91.60% <100.00%> (ø)
ietf/doc/views_charter.py	88.72% <100.00%> (ø)
ietf/doc/views_status_change.py	93.04% <100.00%> (ø)
... and 14 more

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