Modify tree validator

[margalva]

When a tree has multiple levels, it is possible that a top node has "value" empty. That should be accepted.
Check for the validity of 'value' only if it's not empty, or if it's at the last level of the tree hierarchy.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.58%. Comparing base (ead301d) to head (3cf4c7a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #430   +/-   ##
=======================================
  Coverage   81.57%   81.58%           
=======================================
  Files          28       28           
  Lines        7980     7981    +1     
  Branches     1474     1475    +1     
=======================================
+ Hits         6510     6511    +1     
  Misses       1001     1001           
  Partials      469      469           

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