Enhance ?: operator to support null dereference on lhs

[jaccomoc]

The ?: operator will return the right-hand-side value if the left-hand-side is null.
So we can write:

x.a.b.c ?: 'xxx'

This will return 'xxx' if x.a.b.c is null.
But if x.a.b or x.a or even x were null then we would get an error. If we want to cater for these possibly being null we have to write:

x?.a?.b?.c ?: 'xxx'

It would be nice if the ?: would return the right-hand-side value if either the left-hand-side evaluates to null or would throw a NullError if evaluated.
Then, x.a.b.c ?: 'xxx' would return 'xxx' if any of the dereferenced fields were null.