Fix panic when using join_selections_space (#9783)

Joining lines with Alt-J does not properly select the inserted spaces
when the selection contains blank lines. In the worst case it panics
with an out of bounds index.

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
Char index out of bounds: char index 11, Rope/RopeSlice char length 10'

Steps to reproduce:
* Create a new document
    ```
    a

    b

    c

    d

    e
    ```
* % (Select all)
* Alt-J (join and select the spaces)

0 files changed, 0 insertions, 0 deletions