Initialize variable

The `mount_monitor` variable is only set if the boolean
`with_mount_monitor` variable is set to TRUE, but the compiler does not
know that, so it'll warn when calling `g_clear_object()` even if the
clearing operation is gated with the same boolean.

Initializing with NULL does not cost us anything, and eliminates a
conditional branch.

https://bugzilla.gnome.org/show_bug.cgi?id=794732