It's possible for the user to have selected nothing, and trying to
operate on the nil highlight can cause confusion or crashes. This
restores the behaviour before commit 7a0e3d5e68 ("readerhighlight:
remove selected_word and use selected_text everywhere"), which missed
this case.
In addition, add some debug guards to ReaderHighlight methods which
cannot handle selected_text being nil (or at least, shouldn't be called
with selected_text being nil).
Fixes: 7a0e3d5e68 ("readerhighlight: remove selected_word and use selected_text everywhere")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
3fd931bb2f