Introduce psa_key_handle_is_null inline function

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/suites/test_suite_pk.function b/tests/suites/test_suite_pk.function
index 22bf0e7..5fee0d7 100644
--- a/tests/suites/test_suite_pk.function
+++ b/tests/suites/test_suite_pk.function
@@ -158,7 +158,7 @@
     mbedtls_pk_init( &pk );
 
     key = pk_psa_genkey();
-    if( key == 0 )
+    if( psa_key_handle_is_null( key ) )
         goto exit;
 
     TEST_ASSERT( mbedtls_pk_setup_opaque( &pk, key ) == 0 );