Fix nullptr dereference in NvmlState

[cwharris]

Fixes a bug where NvmlState was dereferencing a nullptr when libnvidia-ml.so.1 is not available.

[codecov]

Codecov Report

Merging #319 (786ce10) into branch-23.07 (810a7a6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@              Coverage Diff              @@
##           branch-23.07     #319   +/-   ##
=============================================
  Coverage         73.21%   73.22%           
=============================================
  Files               390      390           
  Lines             13379    13380    +1     
  Branches           1008     1010    +2     
=============================================
+ Hits               9796     9797    +1     
  Misses             3583     3583           

Flag	Coverage Δ
cpp	69.04% <100.00%> (-0.01%)	⬇️
py	42.23% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
cpp/mrc/src/internal/system/device_info.cpp	56.09% <100.00%> (+0.54%)	⬆️

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[cwharris]

/merge