[settings] Treat an empty formatted value as meaning "delete setting"

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

 	int check_len;
 	int rc;
 
-	/* NULL value implies deletion */
-	if ( ! value )
+	/* NULL value or empty string implies deletion */
+	if ( ( ! value ) || ( ! value[0] ) )
 		return delete_setting ( settings, setting );
 
 	/* Parse formatted value */