Fix: htmlentities() shouldn't be called with null (#218)

* Fix: htmlentities(): Passing null to parameter 1 ($string) of type string is deprecated

Author: fedeisas

src/Liquid/StandardFilters.php

@@ -201,6 +201,10 @@ public static function escape($input)
 			return $input;
 		}
 
+		if (is_null($input)) {
+			return '';
+		}
+
 		return htmlentities($input, ENT_QUOTES);
 	}
 

tests/Liquid/StandardFiltersTest.php

@@ -231,6 +231,7 @@ public function testEscape()
 		$data = array(
 			"one Word's not" => "one Word's not",
 			"&><\"'" => "&amp;&gt;&lt;&quot;&#039;",
+			null => '',
 		);
 
 		foreach ($data as $element => $expected) {