Introduce psa_key_handle_is_null inline function

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 9494c65..391e93c 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -63,7 +63,7 @@
         return( 1 );
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
-    if( conf->psk_opaque != 0 )
+    if( ! psa_key_handle_is_null( conf->psk_opaque ) )
         return( 1 );
 #endif /* MBEDTLS_USE_PSA_CRYPTO */