Remove trailing empty ListSweep from merged sweep

[dstrain115]

• when calling merge_single_qubit_gates_to_phxz_symbolized, if remaining symbols is empty, then an empty ListSweep is added to the end of the sweep.
• This empty ListSweep confuses parsers, which then interpret this as a sweep of zero length and return an empty list.
• This now only adds this ListSweep if there is something to add.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.37%. Comparing base (23afd92) to head (04a6ff4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7664   +/-   ##
=======================================
  Coverage   99.37%   99.37%           
=======================================
  Files        1082     1082           
  Lines       96689    96691    +2     
=======================================
+ Hits        96082    96088    +6     
+ Misses        607      603    -4     

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