fix: User event invoice endpoint

[iamareebjamal]

For fossasia/open-event-frontend#5049

[codecov]

Codecov Report

Merging #7270 into development will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           development    #7270   +/-   ##
============================================
  Coverage        63.56%   63.56%           
============================================
  Files              259      259           
  Lines            13038    13038           
============================================
  Hits              8288     8288           
  Misses            4750     4750           

Impacted Files	Coverage Δ
app/api/schema/users.py	100.00% <100.00%> (ø)
app/models/event_invoice.py	95.23% <100.00%> (ø)

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