Fix admin endpoint permissions, nullable columns

[didoda]

This fixes a buggy behaviour in "Administration -> Endpoint Permissions" page: NULL values were displayed as first value of the correspondant dropdown. With this, - is shown when value is null.

An example:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.69%. Comparing base (c701de4) to head (0c8c7ef).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1168   +/-   ##
=========================================
  Coverage     99.69%   99.69%           
- Complexity     1301     1304    +3     
=========================================
  Files            97       97           
  Lines          4986     4999   +13     
=========================================
+ Hits           4971     4984   +13     
  Misses           15       15           

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