Fix error 500 when ordering with offline payments

[jarekmajcher]

Fixes #101

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 17.88%. Comparing base (a8c38b6) to head (099be00).
Report is 2 commits behind head on 4.x.

Files with missing lines	Patch %	Lines
src/Message/Handler/SendPurchaseEventHandler.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x     #102   +/-   ##
=========================================
  Coverage     17.88%   17.88%           
  Complexity      163      163           
=========================================
  Files            36       36           
  Lines           699      699           
=========================================
  Hits            125      125           
  Misses          574      574           

Flag	Coverage Δ
	17.88% <0.00%> (?)

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