[FIX] Allowing to render JSON null type

[bclenet]

Closes #2260

Nothing in the specification prevents from using JSON's null as value for metadata fields.
Although it is not rendered properly e.g. in metadata tables.

This PR modifies the get_link function in bids-specification/tools/schemacode/src/bidsschematools/render/utils.py to allow for this kind of rendering (provide a link for the null type):

Key name	Requirement Level	Data type	Description
Command	REQUIRED	string or null	Command used to run the tool. Set to null to describe that the activity was performed manually.

Note

For this to work, src/schema/objects/metadata.yaml must be filled like this:

Command:
  ...
 anyOf:
   type: string
   type: "null"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.83%. Comparing base (373da35) to head (6c01795).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2265   +/-   ##
=======================================
  Coverage   82.83%   82.83%           
=======================================
  Files          20       20           
  Lines        1672     1672           
=======================================
  Hits         1385     1385           
  Misses        287      287           

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