Fix Backend B2B RestHandler

[Sn0w3y]

Fix for: https://community.openems.io/t/exception-in-backend2backend-connection/10220

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

:x: Your patch status has failed because the patch coverage (0.00%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #3211      +/-   ##
=============================================
+ Coverage      59.07%   59.10%   +0.03%     
  Complexity       181      181              
=============================================
  Files           2662     2662              
  Lines         114360   114329      -31     
  Branches        8407     8404       -3     
=============================================
+ Hits           67552    67559       +7     
+ Misses         44278    44242      -36     
+ Partials        2530     2528       -2     

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[sfeilmeier]

I backported the fix in #3218

Details: https://github.com/OpenEMS/openems/pull/3218/files#diff-309dbff3c78f1cd944da321f9a95e29581eebb32e49cd794b68ed832b9e5b30c