Do nothing when pinObject is a nonexistent element

[HerringtonDarkholme]

Because Position normalizes pinObject (as in Position.pin's signature), its element will become a VIEWPORT plain object when pinObject contains a nonexistent element.
This would break the following CSS query in old IE(<=9) if the user is not meticulous enough.

However, passing empty jQuery object is quite pervasive, and it is even harder to preclude pernicious problems like this: $('<div id="must-exist">').appendTo('#yet-another-nonexistent'). jQuery creates a detached div but removes it then because no container is found. The final result is still an empty $ object.

In old IE, querying css property of empty jQuery will produce an exception that freezes all subsequent code in the same context, even if Position.pin is wrapped in try..catch.. block. In modern browsers this does not matter much but still halting futile code execution is preferred.

Position can be more care-free if the burden of validating element is placed not on users. I hope this patch can make it done.

I tried to write new test case in the specs but did not manage to set up the test environment. I only tested on my local project, I wish this revision will pass.

Best wishes

[afc163]

Merged manually, Thx a lot!