Server: Call uv_getaddrinfo with NULL service when no port

When using serverstart("ip.ad.d.r:") to listen on a random port, we need
to abide by getaddrinfo()'s API and pass in a NULL service, rather than
an empty string.

When given an empty string, getaddrinfo() is free to search for a
service by the given name (since the string isn't a number) which will
fail.  At least FreeBSD does perform this lookup.

src/nvim/event/socket.c

@@ -45,6 +45,12 @@ int socket_watcher_init(Loop *loop, SocketWatcher *watcher,
       return UV_EINVAL;
     }
 
+    if (*port == NUL) {
+      // When no port is given, (uv_)getaddrinfo expects NULL otherwise the
+      // implementation may attempt to lookup the service by name (and fail)
+      port = NULL;
+    }
+
     uv_getaddrinfo_t request;
 
     int retval = uv_getaddrinfo(&loop->uv, &request, NULL, addr, port,