Added default empty string to placeholder variable

php 8.1, eeh rating module was throwing:

```Deprecated
htmlentities(): Passing null to parameter #1 ($string) of type string is deprecated

ee/ExpressionEngine/View/_shared/filters/filter.php, line 17```

Basically, the collection filter didn't have a default specified, so it was null and htmlentities didn't love it.

Seemed better to put a default here than require it of add-ons.

system/ee/ExpressionEngine/Service/Filter/Filter.php

@@ -58,7 +58,7 @@ abstract class Filter
     /**
      * @var string The value to use for the custom input's placeholder="" attribute
      */
-    protected $placeholder;
+    protected $placeholder = '';
 
     /**
      * @var bool Whether or not this filter has a custom <input> element