Fixes #36703

What?

This PR fixes an incorrect behaviour where, when deleting a value from the Color Picker's detailed inputs (e.g. the R value in RGB), all other inputs (e.g. the G and B in RGB) would be reset to 0.

Why?

Deleting a number field is a common user action. Resetting unrelated fields is an unexpected side effect with no obvious workarounds.

How?

The proposed change normalizes the onChange input, so that it's converted to 0 whenever it's an empty string or undefined.

The change is larger than strictly needed because of type discrepancies detected by TypeScript.

cc @mirka @ciampo:
I would have loved to remove the // @ts-expect-error TODO: Resolve discrepancy in NumberControl, which I think is reasonably resolved here.
Though, removing it triggers a new error with the return type, which probably requires more familiarity with TS than I have.

EDIT: in e8a99e6 I've tightened up the type checks (thanks to @ciampo for pointing out that the error was on the param, not on the return as I thought!), which allowed me to remove the linter exception. Let me know if this is a reasonable approach!

Testing Instructions

1. Open a Color Picker, e.g. select a block and modify its Text color.
2. Toggle Show detailed inputs.
3. Select RGB in the dropdown that appears. (Bug also exists when using HSL.)
4. Type in something like R = 200, G = 200, B = 200.
5. Double click on the R field.
6. Press Delete, as if you were about to modify it to be 100.
7. ⚠️ Ensure the G and B fields keep their values.