🐛 Fix compatibility check in alternating checker

[burgholzer]

Description

Fix an oversight in one of the compatibility checks for the alternating checker that could cause segfaults. An exception was thrown erroneously if both circuits contained ancillary qubits that were idle. While fixing, I took the opportunity to simplify the underlying code and eliminate some code duplication.

Fixes #236

Checklist:

• [x] The pull request only contains commits that are related to it.
• [x] I have added appropriate tests and documentation.
• [x] I have made sure that all CI jobs on GitHub pass.
• [x] The pull request introduces no new warnings and follows the project's style guidelines.

[codecov[bot]]

Codecov Report

Merging #237 (6efd617) into main (7d07e6e) will decrease coverage by 0.2%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #237     +/-   ##
=======================================
- Coverage   97.0%   96.9%   -0.2%     
=======================================
  Files         34      34             
  Lines       1721    1716      -5     
  Branches     210     210             
=======================================
- Hits        1671    1664      -7     
- Misses        50      52      +2     

Flag	Coverage Δ
cpp	96.8% <100.0%> (-0.2%)	:arrow_down:
python	97.3% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/checker/dd/DDAlternatingChecker.cpp	86.8% <100.0%> (-2.1%)	:arrow_down:
src/checker/dd/DDEquivalenceChecker.cpp	90.8% <0.0%> (-1.0%)	:arrow_down:

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