Fix panic when doing various cursor movements with a pending mouse selection (#13016)

This fixes a panic in the `SelectionsCollection::first_anchor` when
there was a pending mouse selection and no other selections. Until
recently, this method was only used in vim mode, but as of
https://github.com/zed-industries/zed/commit/53b0720d546b6541fbf236d2f9d2dadbce6f126e,
it's also used in the normal `move_up` and `move_down` actions.

So until recently, the panic that this fixes could only happen in vim
mode.

Release Notes:

- Fixed a crash that could happen when using certain cursor-motion
bindings with a pending mouse selection.