Change None values to none strings

[nfahlgren]

Describe your changes
In the function plantcv.plantcv.utils.json2csv the unique values for a list of metadata values is computed but fails when one or more of the values is None. This PR changes the None values to a "none" string, which is the default metadata value.

Thanks to @leowlima for reporting the issue!

Type of update
Is this a: Bug fix

Additional context
A better long-term solution would be to set the values to the default value instead of None.

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (339fb09) 99.98% compared to head (33da64b) 99.98%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1442   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         158      158           
  Lines        6973     6975    +2     
=======================================
+ Hits         6972     6974    +2     
  Misses          1        1           

Flag	Coverage Δ
unittests	99.98% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
plantcv/utils/converters.py	100.00% <100.00%> (ø)