refactor: avoid deprecation warnings around extra dims

[romanc]

Description

"extra dims" have been renamed to "data dimensions" in NDSL 1 and after doing so we figured that we need to be careful with mutable default values 2. Togehter, these two PRs make specifying extra_dim_lengths unnecessary if they are the default empty dictionary.

How Has This Been Tested?

All good as long as CI is happy.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included: N/A
• Targeted model if this changed was triggered by a model need/shortcoming: N/A

[twicki]

Looks ready to merge if we want it in before the release