cleanup: Avoid "lonely" ifs where it makes sense

cleanup: Avoid "lonely" ifs where it makes sense

If an else block only contains an if statement, the two can be
combined into an if-else block, which cuts down on indentation
and usually helps legibility.

There are exceptions (for instance where the outer if and else
blocks are mirrored), but where it makes sense, change the code
to avoid lonely ifs.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/805