UBUNTU: [Debian]: dkms-build: Avoid executing post-processor scripts twice

UBUNTU: [Debian]: dkms-build: Avoid executing post-processor scripts twice

BugLink: https://bugs.launchpad.net/bugs/1840704
The regex to generate dkms_build_generic from dkms_build_specific
will return the same script name if the package name does not end
with a dash followed by digits. In this case the script will be
executed twice.

Fix this by returning an empty string if the regex does not match
and skipping execution if the dkms build script name is empty.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>