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>
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index 449cbc5..3a3ee09 100755
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -134,7 +134,7 @@
         set(SUITE ${TEST_PSA_API})
     endif()
 
-    if (NOT DEFINED PSA_API_TEST_TARGET)
+    if (NOT DEFINED PSA_API_TEST_TARGET OR PSA_API_TEST_TARGET STREQUAL "")
         string(REGEX REPLACE ".*/" "" PSA_API_TEST_TARGET ${TFM_PLATFORM})
     endif()