fix: use isset() when checking table property value (#52)

We can't access the `$this->table` property before initialization, so let's use `isset()` to check if the property has a value.

Reviewed-on: https://codeberg.org/vlw/php-mysql/pulls/52

src/MySQL.php

@@ -35,7 +35,7 @@
 		*/
 
 		private function throw_if_no_table() {
-			if (!$this->table) {
+			if (!isset($this->table)) {
 				throw new Exception("No table name defined");
 			}
 		}