docs: Clarify that value 0 is valid when setting contact fields via API

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Adds a note to the Contacts API documentation explaining that Mautic only treats null and empty strings as empty values. The value 0 (integer or string) is a valid value and will be saved to the field. This clarification helps API developers who need to set numeric fields to zero.

Trigger Events

mautic/mautic PR #15900: fix: use strict comparison for isEmpty to allow saving value 0 in contact fields

Promptless Research (1 GitHub PR)

• Reviewed mautic/mautic#15900

Agent Response

I've completed my review of PR #15900 and created a documentation update.

Summary

PR #15900 fixes a bug where setting a contact field value to 0 (zero) via the REST API was silently ignored. The root cause was PHP's loose comparison treating 0 as an empty value. With the fix, 0 is now correctly saved as a valid value.

Documentation Update

I created a suggestion that adds a clarifying note to the Contacts API documentation explaining:

• Only null and empty strings ('') are treated as empty values
• The value 0 (whether integer or string) counts as a valid value and gets saved
• This is relevant when using overwriteWithBlank or setting numeric fields to zero

Suggestion: Clarify that value 0 is valid when setting contact fields via API

File updated: docs/rest_api/contacts.rst

This documentation clarification will help API developers understand the expected behavior when working with numeric contact fields and the overwriteWithBlank parameter.

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Citation: Based on bug fix in LeadModel::setFieldValues() that changed from loose to strict comparison for empty value detection. The PR description explains that null == 0 returned true (buggy) but now null === 0 returns false (correct), allowing 0 to be saved as a valid value.
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