🐛 properly handle repeated cancellation in single-qubit gate fusion

[burgholzer]

Description

This PR fixes a small bug in the single-qubit gate fusion optimisation method where repeated cancellations would lead to empty compound operations that were then directly accessed without checking whether there are actually elements to access.

Checklist:

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

[github-actions]

Cpp-Linter Report ✔️

No problems need attention.

Have any feedback or feature suggestions? Share it here.

[codecov]

Codecov Report

Merging #459 (08bb1a5) into main (b2db9e2) will increase coverage by 0.0%.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #459   +/-   ##
=====================================
  Coverage   89.9%   89.9%           
=====================================
  Files        109     109           
  Lines      11666   11673    +7     
  Branches    2042    2044    +2     
=====================================
+ Hits       10494   10504   +10     
+ Misses      1172    1169    -3     

Flag	Coverage Δ
cpp	89.7% <100.0%> (+<0.1%)	⬆️
python	100.0% <ø> (ø)

Files	Coverage Δ
src/CircuitOptimizer.cpp	85.6% <100.0%> (+0.3%)	⬆️

... and 1 file with indirect coverage changes