make compatibilityChecker robust if v2 api is disabled. dirty fix for #1229

[Lokowandtg]

With this change, the compatibility checker does not fail if v2 API is not available.
In that case, no check is executed, which is not correct.
I will work on adding v3/info endpoint and checking that version next.

[Kehrlann]

Hey @Lokowandtg

Questions:

1. What happens today we have a foundation with v2 disabled? What's the error?
2. You plan to add v3 compatibility, but API versions are different between v2 and v3. What's your plan?

[Lokowandtg]

Without this pull request, the CloudFoundryClientCompatibilityChecker runs into a NullPointerException when v2 API is not available.
The next plan is to check all "v3" integration-tests where some "v2" endpoint is called and see if there is a replacement that uses only v3. So v2-integration-tests should stay as is, v3 should only use v3 and the rest I will check later.