Disable disp generation for some variables

[slacrherbst]

Disables genDisp routine when disp=''. This will help processing large numpy arrays at large rates.

[codecov-commenter]

Codecov Report

Merging #932 (c6aba9f) into pre-release (ac69845) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

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@@               Coverage Diff               @@
##           pre-release     #932      +/-   ##
===============================================
- Coverage        26.07%   26.06%   -0.01%     
===============================================
  Files               71       71              
  Lines             7759     7761       +2     
  Branches          1167     1168       +1     
===============================================
  Hits              2023     2023              
- Misses            5573     5574       +1     
- Partials           163      164       +1     

Impacted Files	Coverage Δ
python/pyrogue/_DataReceiver.py	24.48% <ø> (ø)
python/pyrogue/_Variable.py	60.17% <0.00%> (-0.19%)	⬇️

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