Fix invalid regular expression NixOS#156861

Empty parantheses are not supported in regular expressions on
Darwin/macOS. The old regular expression produces an error during
evaluation. This commit fixes that.

Nix‘s `builtins.match` works with extend POSIX regular expressions. The
specification for these regular expression states[^1] that the result
for a left paranthesis immediately followed by a right paranthesis
outside of a bracket expression is undefined.

[^1]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04_03

nixos/modules/services/networking/kresd.nix

@@ -9,7 +9,7 @@ let
   # On Nix level we don't attempt to precisely validate the address specifications.
   # The optional IPv6 scope spec comes *after* port, perhaps surprisingly.
   mkListen = kind: addr: let
-    al_v4 = builtins.match "([0-9.]+):([0-9]+)()" addr;
+    al_v4 = builtins.match "([0-9.]+):([0-9]+)($)" addr;
     al_v6 = builtins.match "\\[(.+)]:([0-9]+)(%.*|$)" addr;
     al_portOnly = builtins.match "([0-9]+)" addr;
     al = findFirst (a: a != null)