Fix: missing null check in certifyLegal blobstore backend

[robert-cronin]

Description of the PR

Fixes #2192 by adding the missing null check

Verification of fix:

PR Checklist

• [ x ] All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If ent schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[mihaimaruseac]

This looks good to me, but also waiting on @pxp928 for checking that I didn't miss anything

[pxp928]

LGTM!