Remove "repositories" key-must-exist check

An ADT should not have a hard requirement for the top-level `repositories` key. The validation check here can be modified to succeed if no `repositories` are defined.

Relevant code:
https://github.com/micado-scale/micado-parser/blob/main/micadoparser/validations.py#L46

Start at micadoparser/validations.py around line 46 and inspect the validation for the top-level `repositories` key. Adjust the validation so an ADT without defined repositories succeeds, then verify that an ADT with no `repositories` key is accepted while existing repository validation still behaves as expected.

Written by the indexing model from the issue text.