Allow HSes to omit device display names

[DMRobertson]

Hopefully gets matrix-org/synapse#10015 over the line.

I don't speak perl and it's not easy to test this change against that PR because it comes from a fork.

I suggest we merge this if CI is green and move on with our lives.

[DMRobertson]

wtf was I smoking---this = is assinging, not testing (==?)

I'm going to have to read perl docs now, aren't I?

[H-Shay]

My perl is as good as yours but it's green so sure?

[DMRobertson]

I tried to convince myself this was sane as follows:

dmr on titan in sytest on  dmr/relax-device-display-name via 🐪 v5.34.1 
2022-10-18 21:20:16 ✔  $ cat temp.pl
#! /usr/bin/env perl
use strict;
use warnings;
my $var;

print "I think \$var is undef?\n";
if (defined $var) { print "defined\n" } else { print "undefined\n" };
if ($var eq "hello") { print "eq \"hello\"\n" } else { print "not eq \"hello\"\n"};

$var = "hello";
print "\$var is \"hello\"?\n";
if (defined $var) { print "defined\n" } else { print "undefined\n" };
if ($var eq "hello") { print "eq \"hello\"\n" } else { print "not eq \"hello\"\n"};

$var = "world";
print "\$var is \"world\"?\n";
if (defined $var) { print "defined\n" } else { print "undefined\n" };
if ($var eq "hello") { print "eq \"hello\"\n" } else { print "not eq \"hello\"\n"};


dmr on titan in sytest on  dmr/relax-device-display-name via 🐪 v5.34.1 
2022-10-18 21:20:39 ✔  $ ./temp.pl 
I think $var is undef?
undefined
Use of uninitialized value $var in string eq at ./temp.pl line 8.
not eq "hello"
$var is "hello"?
defined
eq "hello"
$var is "world"?
defined
not eq "hello"