Fix bug in Cloner::cloneEmpty()

If an image was cloned with cloneEmpty() and a fully transparent color
for exampe "rgba(255, 255, 255, 0)" the turned out as white background.

This patch sets the background color as fully transparent if it has an
alpha channel value of 0 i.e. 100% transparent (clear).

src/Drivers/Gd/Cloner.php

@@ -67,6 +67,10 @@ public static function cloneEmpty(
         imagealphablending($clone, true);
         imagesavealpha($clone, true);
 
+        if ($background->isClear()) {
+            imagecolortransparent($clone, $processor->colorToNative($background));
+        }
+
         return $clone;
     }