Add default return values for Padding params

gfx/padding.lua

@@ -49,19 +49,19 @@ function Padding:setDirty(fullInvalidate)
 end
 
 function Padding:getPaddingLeft()
-  return self.left
+  return self.left or 0
 end
 
 function Padding:getPaddingRight()
-  return self.right
+  return self.right or 0
 end
 
 function Padding:getPaddingTop()
-  return self.top
+  return self.top or 0
 end
 
 function Padding:getPaddingBottom()
-  return self.bottom
+  return self.bottom or 0
 end
 
 local function compareSet(value, default, didChange)