There are cases where a dendrite instance is unaware of a pseudo ID for
a user, the user is not a member of that room. To represent this case,
we currently use the 'zero' value, which is often not checked and so
causes errors later down the line. To make this case more explict, and
to be consistent with `QueryUserIDForSender`, this PR changes this to
use a pointer (and `nil` to mean no sender ID).
Signed-off-by: `Sam Wedgwood <sam@wedgwood.dev>`
c7193e24d0