🐛 Fix out-of-bounds error in ZX EdgeIterator and needless dummy register in empty circuit construction

[burgholzer]

Description

This PR fixes an out-of-bounds error in the ZX packages EdgeIterator that would happen whenever a ZX diagram would only consist of eliminated nodes.

In addition, this PR fixes an oversight in the QuantumComputation constructor where an empty qubit register "q" was added even on a zero-qubit circuit.

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.1%. Comparing base (7ef1ccb) to head (4ed96d5).
Report is 141 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #758   +/-   ##
=====================================
  Coverage   92.1%   92.1%           
=====================================
  Files        125     125           
  Lines      13770   13775    +5     
  Branches    2157    2160    +3     
=====================================
+ Hits       12684   12690    +6     
+ Misses      1086    1085    -1     

Flag	Coverage Δ
cpp	91.8% <100.0%> (+<0.1%)	⬆️
python	99.7% <ø> (ø)

Files with missing lines	Coverage Δ
include/mqt-core/ir/QuantumComputation.hpp	91.9% <100.0%> (+<0.1%)	⬆️
src/zx/Utils.cpp	83.1% <100.0%> (+1.7%)	⬆️