Nullable types and ternary operators are around for many years now and may be not everybody know that ternary operator doesn't support assigning null values to nullable types.
It's not a personal attack, I'm just pointing out that beyond talking about the compiler error, and that the team made a design decision to disallow it, you didn't go into the reasons why the decision was made.
So if you can understand, the ternary operators where available before the nullable types. So when the ternary operator was designed, it was designed to accept the values (on true and false) to implicitly convertible values. So now the team is however considering to support the same in the future version.
No offense taken :)
From a maintenance and debugging view I would prefer to see the more explicit definition provided in the work around. However I do know some people who would much rather see an explicit If.. Then statements.