Fix the derivative of MottonenStatePreparation where possible

[dwierichs]

Context:
The decomposition of MottonenStatePreparation skips some gates for special parameter values/input states.
See the linked issue for details.

Description of the Change:
This PR introduces a check for differentiability so that the gates only are skipped when no derivatives are being computed.
Note that this does not fix the non-differentiability at other special parameter points that also is referenced in #5715 and that is being warned against in the docs already.
Also, the linked issue is about multiple operations and we here only address MottonenStatePreparation.

Benefits:
Fixes parts of #5715. Unblocks #5620 .

Possible Drawbacks:

Related GitHub Issues:
#5715

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (45f8d4a) to head (48b3ef6).
Report is 251 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5774      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         420      420              
  Lines       40138    39846     -292     
==========================================
- Hits        40010    39716     -294     
- Misses        128      130       +2     

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[astralcai]

Just a small suggestion regarding skipping test cases, otherwise LGTM!

[astralcai]

LGTM! 🎸

[KetpuntoG]

LGTM! happy to approve once I cleared up a couple of doubts

First of all, I would like to make sure. The derivative of a function with real output (as an expected value), can it give a complex number?

[dwierichs]

I answered both the complex numbers and precision issues in the comment :)

[KetpuntoG]

Thanks for the clarifications! Ready to go 🚀