[llvm][utils] Handle Issue/PR authors having no display name set (#186094)
user.login is the account name and user.name is an optional display name
(often their full name). I got an email generated from a colleague's PR
that said:
```
Author: None (<their username>)
```
As they hadn't set user.name in their account. Which isn't a problem,
but it would be neater if we didn't print None.
So I've added a helper function to handle that. If the user has set
both, the output is as it was before, if the user has not, we just show
the login name.
The login name can apparently be None too. In that case we'll print
"None" for it. This does not seem to be a common case though, and I'm
not sure printing anything else would be any more useful.