BUG: add a newline at end of filestore json file

[tangkong]

Description

Add a blank newline whenever writing the filestore .json file

Motivation and Context

Stumbled into this. The json would fail pre-commit checks every time we tried to commit it. While infrequent this is something that's easily avoided.

Without this, we would fail pre-commit

How Has This Been Tested?

Interactively

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate