fix serializer alert rule projects

[nhsiehgit]

Serializer was incorrectly serializing projects for alert rules.

• we were improperly attempting to add an iterable to a set
• we were improperly catchign an exception IF the .get() of a model failed and passing
• We had no tests to validate the serialization

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.70%. Comparing base (6dd1e81) to head (273d23e). Report is 12 commits behind head on master.

:exclamation: Current head 273d23e differs from pull request most recent head af0c4e7. Consider uploading reports for the commit af0c4e7 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #69276 +/- ## ======================================= Coverage 79.70% 79.70% ======================================= Files 6430 6430 Lines 285944 285943 -1 Branches 49292 49295 +3 ======================================= + Hits 227900 227909 +9 + Misses 57607 57597 -10 Partials 437 437 ``` | [Files](https://app.codecov.io/gh/getsentry/sentry/pull/69276?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry) | Coverage Δ | | |---|---|---| | [src/sentry/api/serializers/models/alert\_rule.py](https://app.codecov.io/gh/getsentry/sentry/pull/69276?src=pr&el=tree&filepath=src%2Fsentry%2Fapi%2Fserializers%2Fmodels%2Falert_rule.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry#diff-c3JjL3NlbnRyeS9hcGkvc2VyaWFsaXplcnMvbW9kZWxzL2FsZXJ0X3J1bGUucHk=) | `98.98% <100.00%> (-0.01%)` | :arrow_down: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/getsentry/sentry/pull/69276/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry)