Fix bounded trie merge when merging on empty trie

[rohitsinha54]

When merging on empty trie calculate delta based on other.

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[rohitsinha54]

R: @robertwb

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[rohitsinha54]

I believe we will need to cherry-pick this to 2.62 release branch? (Although since there is no internal use we can choose not to but we can't stop users from using it)

[robertwb]

LGTM once formatting is fixed (run yapf -irp [changed files])

+1 to cherry picking for correctness, even if it's not widely used yet.

[rohitsinha54]

@robertwb I do not have write access to repo and hence can't merge. Can you help me merge this PR? Thanks.