kbuild: deb-pkg: use default string when variable is unset or null

${DEBFULLNAME-${user}} falls back to ${user} when DEBFULLNAME is unset.
It is more reasonable to do so when DEBFULLNAME is unset or null.

Otherwise, the command:

  $ DEBFULLNAME= make deb-pkg

will leave the name field blank.

The same applies to KBUILD_BUILD_USER and KBUILD_BUILD_HOST.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>