fix: blank=True for xml_version

[jennifer-richards]

Required to allow submissions without an xml version to be modified in the admin. The field was already nullable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.76%. Comparing base (ea8377d) to head (edab0fb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9285      +/-   ##
==========================================
+ Coverage   88.74%   88.76%   +0.01%     
==========================================
  Files         320      320              
  Lines       41674    41674              
==========================================
+ Hits        36985    36992       +7     
+ Misses       4689     4682       -7     

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