Allow contract verification tests to have no contracts available

[JamesBurnsCanva]

Note: This issue is written from the perspective of consuming the Java libraries.

Currently when a contract verification test has no contract available, it throws a ContractCaseConfigurationError, with contractCaseErrorCode = "UNDOCUMENTED".
This means that, short of parsing the error message, you cannot determine if a contract verification test has no contracts.

I believe contracts should be evolved on the following principles:

• a provider / verification and a consumer / definition should not be modified in the same PR
• a contract should not be added merged if there is nothing to verify it

Following from these principles, this means that a provider must have its contract verification test merged before any consumers create contract definition tests.
In this case, at the time of the verification test first being merged, we expect there to be no contracts defined against it.
Given that, we should support verification tests which have no contracts available.

[TimothyJones]

I agree, let's support this.

The reason it's currently an error is that it would be easy to set it up with no contracts, and that's often (but not always) a mistake. I'll make it emit a warning instead of failing.