Apparently, neither our previous address parsing code nor
Email::Address::XS recognizes local, username-only addresses
in the form of <username> (without "@host"). Without
this change, Email::Address::XS->address would return
"undef", so we need to filter it out via "grep { defined }"
It seems the cases where users email each other on the same
machine is small and public-inbox won't be able to index
addresses for those cases... Oh well :/