fix: escape nulls in XML api responses

[jennifer-richards]

Replaces null characters with \0 in tastypie XML responses. This should stanch the flow of 500 errors we get when fuzzers hit tastypie endpoints. (Tastypie is correctly rejecting the requests with the garbage, then hitting an exception when it tries to respond with an XML format error that includes the garbage, so these errors are pure noise.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.74%. Comparing base (ea8377d) to head (163d545).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9283      +/-   ##
==========================================
- Coverage   88.74%   88.74%   -0.01%     
==========================================
  Files         320      320              
  Lines       41674    41685      +11     
==========================================
+ Hits        36985    36992       +7     
- Misses       4689     4693       +4     

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