Fix NPE in string function on DatabricksConfig

[pietern]

Changes

If the configuration isn't resolved, the env field will be null, and the toString function would throw an NPE. This change fixes that and adds a few tests. I changed the string output to always be non-empty, even if no properties are set.

Tests

Unit tests.

[tanmay-db]

If my understanding is correct, then this would only happen for when we do config.toString() since we directly call debugString, for the other case (where we call debugString in makeNicerError) we have a check that checks for the null env:

    if (cfg.getEnv() != null) {
      debugString = debugString(cfg);
    }

Since we are putting a null check at the base (in debugString), we might have to remove this check in makeNicerError and update the if condition on line 189, since debugString would never be empty: !debugString.isEmpty()

Otherwise the message that we get from makeNicerError won't contain Env: <none>, Config: <empty>

[mgyucht]

I don't think we need to change anything about that other callsite. Basically, it's saying that we won't include the config in the makeNicerError message if it is not yet resolved, but we can still include that in DatabricksConfig's toString()

[mgyucht]

I don't think we need to change anything about that other callsite. Basically, it's saying that we won't include the config in the makeNicerError message if it is not yet resolved, but we can still include that in DatabricksConfig's toString()