Palette does not scroll with page when Toolbar module not installed

The palette block doesn't seem to scroll when Toolbar module is not enabled. Oddly this doesn't affect D6 sites, despite the fact that the exact same line is in the D6 version but no D6 sites have Toolbar module installed. It's probably a difference between jQuery versions that earlier versions would return a height() value of 0 for non-existant elements, but newer versions return NaN (not a number).

This patch prevents the breakage by only adding the number if something was returned.