Skip command table node set on empty value

[markheik]

This PR fixes an issue where LabOne error log showed that command table cannot be uploaded before ELF, even though no command table is not defined.

Currently when command table file path value is empty, an empty command table is sent to the device. Historically this was used to reset the command table.

However, this produces an error in LabOne DataServer log, that ELF must be loaded first, even though a command table was never meant to be uploaded. If command table must be reset, it should be done via appropriate node: ../commandtable/clear

---

It also ensures that enumerated node values are called with an integer, not string

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (45aeed3) 98.66% compared to head (cb1e284) 98.67%.

Files	Patch %	Lines
src/zhinst/labber/driver/base_instrument.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   98.66%   98.67%           
=======================================
  Files          12       12           
  Lines        1126     1128    +2     
=======================================
+ Hits         1111     1113    +2     
  Misses         15       15           

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[tobiasah]

LGTM