fix redshift RV input float parsing

[kecnry]

Description

This pull request fixes a UI bug when entering RV manually. This was inadvertently introduced in #1036 (953d9d1). The change to use FloatHandleEmpty instead of Any requires string->float casting to be done within JS.

Checklist for package maintainer(s)

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• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set? Milestone is only currently required for PRs related to Imviz MVP.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.89%. Comparing base (ddd3c25) to head (7208b69).
Report is 2599 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1086   +/-   ##
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  Coverage   75.89%   75.89%           
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  Files          82       82           
  Lines        6278     6278           
=======================================
  Hits         4765     4765           
  Misses       1513     1513           

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[ibusko]

Simple enough fix. I checked in the demo notebook and it works fine.

[kecnry]

No idea why CI is failing, seems to be unrelated to the changes here though - @pllim?

[pllim]

Might be another manifestation of something I am trying to fix at astropy/astropy#12875 . Okay to ignore here for now.

[pllim]

Thanks!