exclude null or empty users

[octavioamu]

Description

Remove empty users from query making the search filter to fail on new bounty invite

Refers/Fixes

#5090 (comment)

Testing

locally tested

[codecov]

Codecov Report

Merging #5136 into master will decrease coverage by 0.4%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #5136      +/-   ##
=========================================
- Coverage   30.91%   30.5%   -0.41%     
=========================================
  Files         219     219              
  Lines       17687   18228     +541     
  Branches     2438    2614     +176     
=========================================
+ Hits         5468    5561      +93     
- Misses      11994   12428     +434     
- Partials      225     239      +14

Impacted Files	Coverage Δ
app/dashboard/views.py	15.15% <0%> (+0.73%)	⬆️

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