Fix cached false condition

Since we're using (string) '0' to cache the false condition, a strict equality check won't work here.

1. When we create a coauthor, we check that the slug is not already taken. Since it's not, we cache '0'.
2. Next time we need to get that coauthor `false !== ( $retval = wp_cache_get( $cache_key, self::$cache_group ) )`. Since `false !== '0'`, that passes. Since '0' is not an object, we return false. The cache will never have a chance to get fixed.

We don't want to use the cache if it's any version of false (0, '0', false).

Ref: 6a9f4c2

php/class-coauthors-guest-authors.php

@@ -779,7 +779,7 @@ function get_guest_author_by( $key, $value, $force = false ) {
 
 		$cache_key = $this->get_cache_key( $key, $value );
 
-		if ( false == $force && false !== ( $retval = wp_cache_get( $cache_key, self::$cache_group ) ) ) {
+		if ( false == $force && false != ( $retval = wp_cache_get( $cache_key, self::$cache_group ) ) ) {
 			// Properly catch our false condition cache
 			if ( is_object( $retval ) )
 				return $retval;