Admin: Support Py 3.13

[bblommers]

The SES changes are due to a logic bug that only manifests in Python3.13.

When no CC/BCC fields are set, the fieldvalues would be set to something like: ['to@example.com, foo@example.com', '', '']
Sending that to the builtin emails.utils.getaddresses-method in Python 3.12 results in:
[('', 'to@example.com'), ('', 'foo@example.com'), ('', '')]

But in Python 3.13, this results in an empty list.

This PR ensures that there are no empty strings in fieldvalues, which in turns means that the getaddresses-method keeps working.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.37%. Comparing base (459be00) to head (fda9266).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7984   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files        1125     1125           
  Lines       96356    96356           
=======================================
  Hits        90935    90935           
  Misses       5421     5421           

Flag	Coverage Δ
servertests	28.93% <0.00%> (ø)
unittests	94.34% <100.00%> (+<0.01%)	⬆️

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[github-actions]

This is now part of moto >= 5.0.13.dev54

[cas--]

We encountered this bug in our CI pipeline where different 3.10 point releases have flip-flopped between success (3.10.11, 3.10.13, 3.10.14) and failure (3.10.12, 3.10.15) based on this getaddresses call.

So for reference this actually resolve a problem with all the latest Python versions since they now all default to a new strict=True argument based around this bug fix: python/cpython#102988

• Python 3.8.20 onwards
• Python 3.9.20 onwards
• Python 3.10.15 onwards
• Python 3.11.10 onwards
• Python 3.12.16 onwards
• Python 3.13

[bblommers]

TIL! Thanks for sharing @cas-- - I had no idea this was all due to a CVE.