Fix: soong_ui.bash's wrong check for TOP variable

soong_ui.bash defines `gettop` function and it finds top of the source
tree. It also checks if `TOP` is properly set and if set use it.
However its check was using the wrong operator (`-z`) for it.

Test: m -j
Change-Id: Ie86dc340a9f89f45ff0828ea43f6afa3713bda76