DM-42217: Add unit tests for RBTransiNetConfig

[kfindeisen]

This PR allows RBTransiNetConfig.modelPackageName to be set to either None or the empty string when modelPackageStorageMode == "butler"; previously only None was allowed. Unit tests have been added for all three cases.

[NimSed]

In practical scenarios, when does RBTransiNetConfig.modelPackageName == "" happen?

[kfindeisen]

It happens if you try to unset modelPackageName in a pipeline YAML:

config:
    modelPackageStorageMode: butler
    modelPackageName: ""

I did that at first (that's how I noticed the behavior!) before moving to a Python config file for the shared-code benefits.

[NimSed]

I'd like to have @TallJimbo's opinion here, since he was suggesting the user should not be allowed to specify anything at all for modelPackageName in the yaml -- hence the check and the consequent error message. I think "" is also "something".

[TallJimbo]

I don't care if you use "" or None as the special value for "empty", and a very slight preference to just pick one and reject the other.

[kfindeisen]

Fine -- I still think it's a usability problem, but as I've demonstrated on the other PRs, you can work around it by going through Python. I've descoped this PR to just the unit tests.

[NimSed]

All looks good.