APIv4 - Fix setting nullable/required/default_value field metadata

[colemanw]

Overview

This fixes wonky required fields in Afform by cleaning up getFields metadata.

Before

• Create an afform and add a bunch of checkbox fields like "Do Not Email", "Do Not Phone", "Is Active", etc.
• Afform will make a bunch of them required on the form, seemingly at random.

After

Non-required fields not required by default.

Technical Details

The heart of the bug was the old PHP "0" == FALSE. When it checked to see if a field had a default value it would discount the number "0". This problem wasn't too apparent until we merged #22954 etc. which got misinterpreted by api getFields as making most boolean fields required.

[civibot]

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