Code Modernization: Fix null to non-nullable deprecations in WP_Meta_Query::get_sql_for_clause().

In the `WP_Meta_Query::get_sql_for_clause()`, the `'value'` index from a meta query array is passed to the PHP native `trim()` function without prior validation.

In PHP 8.1, this could lead to a `trim(): Passing null to parameter #1 ($string) of type string is deprecated` notice.

`trim()` expects a text string and is only useful when ''passed'' a text string as no other variable type can contain whitespace.

Fixed now by verifying that the ''value'' is a string before processing it with `trim()`.

This issue is already covered by the existing `Tests_Meta_Query::test_null_value_sql()` and the `Tests_Meta_Query::test_convert_null_value_to_empty_string()` tests.

Follow-up to [17699], [29887], [29940].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51797 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-meta-query.php

@@ -691,7 +691,7 @@ class WP_Meta_Query {
 				if ( ! is_array( $meta_value ) ) {
 					$meta_value = preg_split( '/[,\s]+/', $meta_value );
 				}
-			} else {
+			} elseif ( is_string( $meta_value ) ) {
 				$meta_value = trim( $meta_value );
 			}