[Bug] Adding and then removing a funding amount causes work to be lost

[chiaberry]

Associated issues

closes https://github.com/cityofaustin/atd-data-tech/issues/17596

Testing

URL to test:

https://deploy-preview-1356--atd-moped-main.netlify.app/moped/projects

Steps to test:

Go to the funding tab of a project and select Add a new funding source In the row, choose a Source and then enter 100 in the Amount field but then delete the amount so it is blank. see that it saves correctly.

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Ship list

• [x] Code reviewed
• [x] Product manager approved
• [ ] ~Added to QA test script if applicable~

[mddilley]

thanks again @chiaberry - merging this to wrap on QA testing! 🚀