api: allow creation of a register with optional value

[happybeing]

Description

Makes the value passed to register create functions optional.

@b-zee I think I had this in an earlier PR so maybe the idea was rejected, but if not here it is.

Related Issue

Fixes #<issue_number> (if applicable).

Type of Change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist

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• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have updated the documentation accordingly.
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