fix: Prevent newExporter crash if tree.ndb is nil (backport #622)

[mergify]

This is an automatic backport of pull request #622 done by Mergify.
Cherry-pick of 2777659 has failed:

On branch mergify/bp/release/v0.19.x/pr-622
Your branch is up to date with 'origin/release/v0.19.x'.

You are currently cherry-picking commit 2777659.
  (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:   CHANGELOG.md
	modified:   Makefile
	modified:   benchmarks/cosmos-exim/main.go
	modified:   export_test.go
	modified:   immutable_tree.go
	modified:   import_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   export.go

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

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[tac0turtle]

ah dam, actually we should have not broken api here