ref(rules): Handle improperly formatted rule

[ceorourke]

One customer has a couple improperly formatted alert rules that are causing this error: https://sentry.sentry.io/issues/5245475486/. I've already edited the serializer so it's not possible to get into this state again (we don't save data with a value of "") and had support reach out to the customer to address this, but this issue keeps happening so we can just handle the empty string.

[saponifi3d]

nit: it'd be nice to add a test case for this in the future as well (since we've seen it come up in prod already)

[ceorourke]

nit: it'd be nice to add a test case for this in the future as well (since we've seen it come up in prod already)

I can add one, but there is no future possibility of this happening for new rules, it's just this one org's 2 rules.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.00%. Comparing base (4b1e575) to head (f9e5bb1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #70797      +/-   ##
==========================================
- Coverage   80.00%   80.00%   -0.01%     
==========================================
  Files        6507     6507              
  Lines      290833   290855      +22     
  Branches    50123    50132       +9     
==========================================
+ Hits       232686   232703      +17     
- Misses      57711    57716       +5     
  Partials      436      436              

Files	Coverage Δ
src/sentry/rules/conditions/event_frequency.py	92.36% <100.00%> (+0.05%)	⬆️

... and 14 files with indirect coverage changes

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ IntegrityError: UniqueViolation('duplicate key value violates unique constraint "auth_user_username_key"\nDETAIL:... pytest.runtest.protocol tests/snuba/rules/condi... View Issue

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