tailcfg: add missing omitempty annotation to PopBrowserURL

Change-Id: I8e752afd5bf009c17aae1b53650479b37c3232bd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f