Followup 5d7673eb: still adjust width/height when positioning fails

ve.ui.Context#updateDimensions does two things: it updates the position
of the popup by measuring where the selection is, and it updates the
width and height of the popup by measuring the contents of the inspector
or the context menu.

In some cases, measuring where the selection is fails, and
ve.ce.Surface#getSelectionRect returns null. This seems to only happen
1) in phantomjs while running the test suite and 2) when an inspector
is opened and doesn't adjust the selection while opening. I suspect
it has something to do with the selection not having been recently
set or the documentNode not having focus, but I haven't investigated
this in depth yet.

Prior to 5d7673eb, the editor would simply crash of the position
measurement failed. 5d7673eb changed this to just bail completely if
the position couldn't be measured, but that's also wrong. Without a
position measurement, we can't update the position of the popup, but
we can still adjust its width and height, and we should.

The failure to adjust the popup's width and height in this case was
causing a bug where opening the link inspector with the selection
set exactly to what the inspector would expand it to would cause the
popup's width and height to never be adjusted, and most of the inspector
to be invisible.

Bug: 58301
Change-Id: Ia6504e0a7b0cd1affbff2632e9f573684f1d5dcb