fix: handle invalid fields properly in proc material upload form

[jennifer-richards]

The validation in the UploadProceedingsMaterialForm incorrectly handled invalid or omitted fields or files during validation. This corrects that handling and adds tests.

This should prevent server errors when uploading files with incorrect file types / extensions. The form assumed cleaned_data['file'] would exist if a file was uploaded, but Django removes invalid data from cleaned_data.

[codecov]

Codecov Report

Merging #3784 (1ad521b) into feat/bs5 (8bd4851) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 1ad521b differs from pull request most recent head 545056b. Consider uploading reports for the commit 545056b to get more accurate results

@@            Coverage Diff            @@
##           feat/bs5    #3784   +/-   ##
=========================================
  Coverage     87.97%   87.98%           
=========================================
  Files           296      297    +1     
  Lines         38795    38804    +9     
=========================================
+ Hits          34130    34141   +11     
+ Misses         4665     4663    -2     

Impacted Files	Coverage Δ
ietf/meeting/views.py	90.73% <100.00%> (+0.02%)	⬆️
ietf/meeting/views_proceedings.py	91.92% <100.00%> (+1.24%)	⬆️
ietf/meeting/utils.py	90.27% <0.00%> (-0.29%)	⬇️
ietf/meeting/models.py	86.08% <0.00%> (ø)
ietf/meeting/templatetags/editor_tags.py	100.00% <0.00%> (ø)

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