Modify IdentityGate to reject input when num_qubits is 0

[ayshiff]

• Updated IdentityGate to reject input with a message instead of raising an error when num_qubits is set to 0.
• Added corresponding test case in identity_test.py to verify the new behavior.

Fixes #6768

[daxfohl]

This could adversely affect some decompositions. Like I think PauliSumExponential(I(q)) decomposes to an I on zero qubits.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (06f12b9) to head (a9fb6e0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6884   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files        1084     1084           
  Lines       94225    94294   +69     
=======================================
+ Hits        92215    92284   +69     
  Misses       2010     2010           

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