~It seems this field is null when the “Require pull request reviews before merging” branch protection setting is disabled.~

I got a reply from GitHub Support and this is what they said:

Thanks for reaching out! The reviewDecision field will be null if it is not in one of the following states:

REVIEW REQUIRED
CHANGES REQUESTED
APPROVED
So basically, if the pull request does not require a review by default (not a protected branch), and there has not been one requested or submitted yet.

I hope this helps! Please let us know if you have further questions and we will be happy to assist.