Allow event blank and fix 500 bug

[jonasdeluna]

Description

Fixes perm checks

Testing

• The code quality is at a minimum required level of quality, readability, and performance.
• I have thoroughly tested my changes.

Please describe what and how the changes have been tested, and provide instructions to reproduce if necessary.

Resolves ABA-1430

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: lego/apps/surveys/permissions.py

Function	Unhandled Issue
has_perm	AttributeError: 'NoneType' object has no attribute 'registrations' /ap... Event Count: 33

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[linear]

ABA-1430 500 error when fetching survey templates

[eikhr]

Looks good👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.97%. Comparing base (82c74d1) to head (f36893a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3832   +/-   ##
=======================================
  Coverage   89.97%   89.97%           
=======================================
  Files         761      762    +1     
  Lines       24364    24370    +6     
=======================================
+ Hits        21921    21927    +6     
  Misses       2443     2443           

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