refactor: replace NullComm with LocalComm

[romanc]

Description

Replace NullComm with LocalComm in pyFV3 since NullComm was deprecated in NDSL version 2025.11.00 in favor LocalComm. See NOAA-GFDL/NDSL#318 for context.

/cc @FlorianDeconinck

How Has This Been Tested?

All good as long as CI is still 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