bgpd: Do not start BGP session if BGP identifier is not set (backport #17959)

[mergify]

Closes #17955

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This is an automatic backport of pull request #17959 done by Mergify.

[mergify]

Cherry-pick of c9a2928 has failed:

On branch mergify/bp/stable/10.0/pr-17959
Your branch is up to date with 'origin/stable/10.0'.

You are currently cherry-picking commit c9a292895.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   doc/user/bgp.rst

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 739f2b5 has failed:

On branch mergify/bp/stable/10.0/pr-17959
Your branch is ahead of 'origin/stable/10.0' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 739f2b566.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   bgpd/bgp_network.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   bgpd/bgp_fsm.c
	both modified:   bgpd/bgpd.h

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[ton31337]

Replaced by #18017.