Replace an NPE with a human-readable error message.

[matanlurey]

See flutter/flutter#152194 where I got befuddling error messages:

Unhandled exception:
Null check operator used on a null value
#0      DeviceLabTestOwner.getTestOwnership (package:cocoon_service/src/request_handlers/test_ownership.dart:60:59)
#1      getTestOwnership (package:cocoon_service/src/request_handlers/flaky_handler_utils.dart:301:20)
#2      validateOwnership (package:cocoon_service/src/foundation/utils.dart:225:27)
#3      remoteCheck (file:///b/s/w/ir/x/w/cocoon/app_dart/bin/validate_task_ownership.dart:31:40)
<asynchronous suspension>
#4      main (file:///b/s/w/ir/x/w/cocoon/app_dart/bin/validate_task_ownership.dart:63:23)
<asynchronous suspension>

[reidbaker]

Did you want to link to a hash instead of https://github.com/flutter/cocoon/blob/main/CI_YAML.md?

[matanlurey]

Done!