refactor: simplify error handling for multipart form reading

[alexandear]

If err is nil, form can't be nil.

If err is nil and form is nil, there will be a panic on the line:

			Message:  "cannot read multipart form: " + err.Error(),

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.77%. Comparing base (1a01022) to head (7b2fa2c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #673   +/-   ##
=======================================
  Coverage   92.77%   92.77%           
=======================================
  Files          22       22           
  Lines        4915     4915           
=======================================
  Hits         4560     4560           
  Misses        308      308           
  Partials       47       47           

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[danielgtaylor]

Thanks!