Improve querying for terms with falsey names and slugs.

Prior to [38677], `get_term_by()` would always return false if
an empty string were passed as the queried 'name' or 'slug'. The
refactor to use `get_terms()` broke this behavior; inappropriately
imprecise `empty()` checks caused the 'name' or 'slug' clause to be
discarded altogether when fetching terms, resulting in an incorrect
term being returned from the function.

We fix the regression by special-casing truly empty values passed
to `get_term_by()`, and ensuring that `WP_Term_Query` is properly
able to handle `0` and `'0'` term queries.

Props sstoqnov.
Fixes #21760.

git-svn-id: https://develop.svn.wordpress.org/trunk@40293 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -835,6 +835,15 @@ function get_term_by( $field, $value, $taxonomy = '', $output = OBJECT, $filter
 		return false;
 	}
 
+	// No need to perform a query for empty 'slug' or 'name'.
+	if ( 'slug' === $field || 'name' === $field ) {
+		$value = (string) $value;
+
+		if ( 0 === strlen( $value ) ) {
+			return false;
+		}
+	}
+
 	if ( 'id' === $field || 'term_id' === $field ) {
 		$term = get_term( (int) $value, $taxonomy, $output, $filter );
 		if ( is_wp_error( $term ) || null === $term ) {