fix(currency-input): Fix currency input type to not allow null

[diogomateus]

What this PR does

Update Currency input to always return a number, even if empty.

Why is this needed?

Please include relevant motivation and context of why this PR is necessary, sentry/linear/notion/...

Solves:
STO-XXX
EMU-XXX
RVN-XXX

How to test?

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Checklist:

• I reviewed my own code
• The changes align with the designs I received
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• I have attached screenshot(s), video(s) or gif(s) showing that the solution is working as expected
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• I have updated the task(s) status on Linear
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Browser support

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