cmake: fix issue with parsing version file located in `/VERSION`

Fixes: #71384

A VERSION file placed in `/` or `<drive>:\` was accidentally being
picked up during `find_package(Zephyr)`.

This happened because Zephyr loads the VERSION file to determine if it
is the correct Zephyr to use.

During initial phase of find_package(), then APPLICATION_SOURCE_DIR is
not defined, causing one version file to be picked up from `/VERSION`
instead of `<app>/VERSION`. `/VERSION` is outside any Zephyr repo, west
workspace, or the application itself and therefore should not be picked
up accidentally.

Fix this be checking that APPLICATION_SOURCE_DIR is defined, and only
when defined, look for a VERSION file there.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>