nixos/matrix-synapse, nixos/mxisd: fix empty usernames

[bachp]

Motivation for this change

It appears #77177 accidently sets the username of the matrix-synapse user to "" (emtpy).
This causes the service fail.

I havent' tested the mxisd service but it seems it will have the same issue, so a fix is also included

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @petabyteboy @worldofpeace

[worldofpeace]

Thanks @bachp

[worldofpeace]

For ref the issue is #77189.