Allow empty PauliVSpace init

[Qottmann]

In situations where one wants to iteratively build up a PaulIVSpace it is handy to be able to initialize an empty PauliVSpace. This is currently not possible and fixed with this PR.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (5b81a8a) to head (8156975).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5675      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         415      415              
  Lines       38886    38596     -290     
==========================================
- Hits        38762    38471     -291     
- Misses        124      125       +1     

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[dwierichs]

Thanks @Qottmann , that's a nice catch!

Would you mind adding your name to the changelog? :)

[Alex-Preciado]

[sc-63068]