Refine behavior of NMDC ingest script

[eecavanna]

On this branch, I made the following two changes:

1. When generating the properties of an Entity, instead of omitting all fields whose values are falsey, only omit the ones whose values are None, [], or {} — in other words, keep those whose values are "" or 0
2. When dumping the JSON data, omit fields whose values are None

CC: @pkalita-lbl and @shreddd

[Copilot]

Pull Request Overview

This PR refines the NMDC ingest script's data handling behavior by improving field filtering logic and JSON serialization. The changes ensure that empty strings and zero values are preserved while removing truly empty containers and null values.

• Modified entity property filtering to preserve empty strings and zero values while still omitting None, empty lists, and empty dictionaries
• Updated JSON serialization to exclude None values from output
• Added cache.json to gitignore for commonly used caching between script runs

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
contrib/nmdc/ingest.py	Updated field filtering logic and JSON serialization to better handle empty vs. falsy values
contrib/nmdc/.gitignore	Added cache.json file to ignore list for temporary data storage

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[pkalita-lbl]

Thanks I meant to follow up on your comment in the other PR. But this is exactly what I was going to suggest doing.