Support NOVALUES parameter for HSCAN

[gerzse]

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Description of change

Issue #3153

The NOVALUES parameter instructs HSCAN to only return the hash keys, without values.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 48.57143% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 91.76%. Comparing base (2f88840) to head (7beaaea).
Report is 19 commits behind head on master.

Files	Patch %	Lines
tests/test_commands.py	34.78%	15 Missing ⚠️
redis/commands/core.py	71.42%	2 Missing ⚠️
redis/_parsers/helpers.py	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3157      +/-   ##
==========================================
- Coverage   91.84%   91.76%   -0.09%     
==========================================
  Files         128      128              
  Lines       33232    33359     +127     
==========================================
+ Hits        30523    30611      +88     
- Misses       2709     2748      +39     

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