fix: Variant::isArray check

[lukasberbuer]

Failed if array is empty.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.72%. Comparing base (7f51d36) to head (3d01ec6).

Files	Patch %	Lines
include/open62541pp/types/Variant.h	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
- Coverage   84.75%   84.72%   -0.04%     
==========================================
  Files          80       80              
  Lines        4212     4216       +4     
  Branches      943      943              
==========================================
+ Hits         3570     3572       +2     
- Misses        172      173       +1     
- Partials      470      471       +1     

Flag	Coverage Δ
Linux	84.72% <88.88%> (-0.04%)	⬇️

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