Introduce PSA_KEY_HANDLE_INIT macro

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 1dc9648..30be676 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -1070,7 +1070,7 @@
     if( ssl->conf->psk_opaque != 0 )
         return( ssl->conf->psk_opaque );
 
-    return( 0 );
+    return( PSA_KEY_HANDLE_INIT );
 }
 #endif /* MBEDTLS_USE_PSA_CRYPTO */