fix: Make currency optional in ticket fees

[iamareebjamal]

For fossasia/open-event-frontend#5107

[codecov]

Codecov Report

Merging #7295 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #7295   +/-   ##
============================================
  Coverage        63.93%   63.93%           
============================================
  Files              259      259           
  Lines            13043    13043           
============================================
  Hits              8339     8339           
  Misses            4704     4704           

Impacted Files	Coverage Δ
app/api/schema/ticket_fees.py	100.00% <ø> (ø)
app/models/event_invoice.py	93.47% <0.00%> (-0.21%)	⬇️
app/api/helpers/utilities.py	70.93% <0.00%> (+1.05%)	⬆️

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