merge r17003 from branch 2.4 to trunk

bug 2705 (related to): if rotate function is called with a 0 (zero) angle,
like plugin RotateImage may do, ExtIM fails with the same error
"convert: invalid argument for option `-rotate': -orient"



git-svn-id: http://piwigo.org/svn/trunk@17004 68402e56-0260-453c-a942-63ccdbb3a9ee

admin/include/image.class.php

@@ -559,6 +559,11 @@ class image_ext_imagick implements imageInterface
 
   function rotate($rotation)
   {
+    if (empty($rotation))
+    {
+      return true;
+    }
+
     if ($rotation==90 || $rotation==270)
     {
       $tmp = $this->width;