Recommit clearing of empty selections in GuiView

A similar fix was reverted (453ce611) because of crashes.
The crashes occurred simply because of a failed check that
we have a buffer view before using it. That is now done in
this commit.

The below commit description is copied from the original
commit (fb05011a):

Empty selections can cause confusing behavior for a few reasons:

(1) some functions behave differently depending on whether there is a
selection. If I press delete, nothing happens (where I expect the
character or inset before the cusor to be deleted). If I toggle bold or
emphasize nothing happens (where if there is no selection the entire
word is toggled). There are other LyX functions that depend on whether
there is a selection or not. Further, I wonder if any part of LyX's code
assumes that if there is a selection it is non-empty.

(2) menu options are incorrectly set. For example, the scissors icon.

For remaining empty selection issues, see #9222.

For more information, see:
https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg184758.html