Fix value of the spectrum returned in the quantum_info.entropies.von_neumann_entropy

[alhajri]

The Von Neumann entropy function when return_spectrum=True returns -log(eigenvalues). However, if the state is pure, return_spectrum=True returned 1.0 which is the value of the eigenvalue, not the -log. The function is now changed to return 0.0 to be more consistent with the behavior of the function in the pure case.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (5676a8a) to head (6a13965).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1360   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          74       74           
  Lines       10759    10759           
=======================================
  Hits        10742    10742           
  Misses         17       17           

Flag	Coverage Δ
unittests	99.84% <100.00%> (ø)

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