Fix param_shift(..., broadcast=True) with zero-length gradient recipes

[dwierichs]

Context:

Description of the Change:
Introduces a special branch for the rare case of zero-length parameter-shift recipes.

Benefits:
Fixes #5622

Possible Drawbacks:

Related GitHub Issues:
Fixes #5622

[sc-62416]

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (v0.36.0-rc0@1798b1f). Click here to learn what that means.

Additional details and impacted files

@@              Coverage Diff               @@
##             v0.36.0-rc0    #5623   +/-   ##
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  Coverage               ?   99.69%           
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  Files                  ?      412           
  Lines                  ?    38336           
  Branches               ?        0           
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  Hits                   ?    38218           
  Misses                 ?      118           
  Partials               ?        0           

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