sd-varlink: make ret optional in sd_varlink_idl_parse()

We have a test failure where the testsuite is calling
sd_varlink_idl_parse() with *ret being NULL. This is now an
assert error. So we could either fix the test or fix the code

Given that it seems genuinely useful to run sd_varlink_idl_parse()
without *ret to e.g. just check if the idl is valid I opted to
fix the code.

src/libsystemd/sd-varlink/sd-varlink-idl.c

@@ -1173,7 +1173,7 @@ _public_ int sd_varlink_idl_parse(
         _cleanup_(sd_varlink_interface_freep) sd_varlink_interface *interface = NULL;
         _cleanup_(varlink_symbol_freep) sd_varlink_symbol *symbol = NULL;
 
-        assert_return(ret, -EINVAL);
+        POINTER_MAY_BE_NULL(ret);
 
         enum {
                 STATE_PRE_INTERFACE,
@@ -1410,7 +1410,8 @@ _public_ int sd_varlink_idl_parse(
         if (r < 0)
                 return r;
 
-        *ret = TAKE_PTR(interface);
+        if (ret)
+                *ret = TAKE_PTR(interface);
         return 0;
 }