If there is no image for the button, return nil.

Fixes #1765 (probably)

Initially, the `DTLinkButton` has a frame size of 0,0. When `minimumHitSize` is set, the frame size is increased to match.  I suspect this is the reason we're sometimes seeing the blue dot, tho I haven't been able to track down the background color is actually being set. 

Checking to see if there is actually an image to display in the button and returning nil when there is not seems to be a viable work around, but we need to test this one where the issue can be reproduced to confirm.