Use non-empty-string instead of string for Provider properties

[westonruter]

The non-empty-string is a more specific type than string. It's clear that this is the intended type. For example, a ProviderMetadata constructor throws an exception if an empty string is provided: !preg_match('/^[a-z0-9\-_]+$/', $id).

This addresses some issues uncovered in WordPress/wordpress-develop#11701.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.12%. Comparing base (6317042) to head (a484e60).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #232   +/-   ##
=========================================
  Coverage     88.12%   88.12%           
  Complexity     1213     1213           
=========================================
  Files            60       60           
  Lines          3934     3934           
=========================================
  Hits           3467     3467           
  Misses          467      467           

Flag	Coverage Δ
unit	88.12% <ø> (ø)

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