jquery.placeholder: Fixup for 7ec7d879ee

Check for 'arguments.length' was done in the wrong scope.
It checked for arguments to .each() which is always 2 (i, el).

This caused a regression where any call to .placeholder() without
arguments resulted in the string 'undefined' to be displayed as
placeholder (since it was doing setAttribute and passing it `text`
which is undefined, and then further using that undefined variable).

Until 7ec7d879ee / a8145d6fa2 this was the only way .placeholder
could be called, so this is a major regression affecting all use
of it.

Change-Id: I862b0f72548ec2122dad9c5d013149ac57688dab