Fix Parameters Behaviour

[tajmone]

Check the syntax behaviour for constant parameters.

I noticed that an empty [u:get ] node was breaking the syntax, but the syntax should be tolerant for a missing constant name because otherwise it uglifies the editing process — I happen to notice this while fast editing multiple lines, and intentionally postponing the constant names.

We want the syntax to handle such cases gracefully.

Also check the behaviour of empty [u:set ] nodes, and add dedicated tests to ensure the fixed behaviour is not broken in the future.