catalog: add NULLIF protection for s.processes division

[leedqin]

Motivation

Notion reported an Evaluation Error: division by zero error in the console coming from mz_console_cluster_utilization_overview view. (https://materializeinc.slack.com/archives/CU7ELJ6E9/p1763742113962939) . I added the NULLIF protection to the s.processes division so that error is not propagated to the environment overview page

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.