Make sure list-like specification keys are correctly normalized and accepted when specified as empty list or none

[happz]

adjust:
adjust: []
link:
link: []
...

There are several such keys, and tmt does not behave consistently when it comes to corner cases. Some null values are accepted, and some are not, and often the null is reported as a mistake by schema while still accepted and normalized nicely. The goal is to define how tmt should behave - should list: null be accepted and treated as list: [] or not? - and then fix schema and normalization to fulfill the said goal.