Build: Take NULL string as NOT DEFINED for PSA_API_TEST_TARGET

This is because the Kconfig system will generate a NULL string
for PSA_API_TEST_TARGET by default, which is equal to "NOT DEFINED".

Change-Id: I3766cb4a08c9d9e9c6f262e3e2df842394e2f58b
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
1 file changed