Source

File: wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php

protected function prepare_value( $value, $schema ) {
		/*
		 * If the value is not valid by the schema, set the value to null.
		 * Null values are specifically non-destructive, so this will not cause
		 * overwriting the current invalid value to null.
		 */
		if ( is_wp_error( rest_validate_value_from_schema( $value, $schema ) ) ) {
			return null;
		}

		return rest_sanitize_value_from_schema( $value, $schema );
	}