Fix port number for lambda config

[arturtamborski]

uhh, sorry :)

broke that here:
6aa6330#diff-651976572b51d1a16ede6a1c4c1be9a1482605ce57ca58fbfca25e4a55c43210L22

AWS lambda does not have PORT configuration:
https://github.com/CodeForPoznan/Infrastructure/blob/master/dev_codeforpoznan_pl_v3.tf#L116

so it failed at runtime:

[ERROR] ValueError: invalid literal for int() with base 10: 'None'
Traceback (most recent call last):
  File "/var/task/backend/handlers.py", line 15, in api
    return handle_request(create_app(), event, context)