Reject an empty body on VWS endpoints which take JSON

[adamtheturtle]

Closes #3550.

An empty body given to POST /targets, PUT /targets/{target_id}, POST /targets/{target_id}/instances or the reco counts report endpoint raised an uncaught JSONDecodeError from validate_keys, because validate_json returned early for an empty body.

This matches real Vuforia instead. I checked each endpoint against the real service with a correctly signed empty body, which differs from the response suggested in the issue:

endpoint	real Vuforia response
POST /targets	500 Fail
PUT /targets/{target_id}	500 Fail
POST /targets/{target_id}/instances	400 BadRequest
POST /imagetargets/databases/{database_id}/reports/recoCounts	400 Fail

validate_json now takes the request method and rejects an empty body on POST and PUT with those responses. GET and DELETE requests are unaffected.

The new test_empty_body runs against real Vuforia as well as the mock backends. Unlike the other malformed bodies which test_invalid_json.py sends, real Vuforia answers an empty body promptly, so it is not in the real-Vuforia skip list. It skips the endpoints which do not take JSON before sending anything, so it costs one request per JSON endpoint. The new test is added to the CI ci_pattern list and passes on all three backends locally.

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