bps l1 processor, fixed invalid computation of in/out-of-band power ratio values

[rpiantanida]

Linked issue

Closes #

• The linked issue is labelled status:approved, good-first-issue or help-wanted.
  Being open in the backlog is not enough on its own: only these three labels mean the scope has been triaged and approved for implementation.

What this PR changes

• When computing in/out-of-band power ratio a 0/0 computation was occurring, resulting in nan annotations in L1 product. This PR fixes this issue

Notes for reviewers

User-facing change

• This PR introduces a user-facing change (API, CLI, output format, performance, documentation).

If yes, write one short release-note sentence here (it will be picked up in CHANGELOG.md):

Documentation

• This PR modifies the documentation (Sphinx site, README, wiki, ATBD, Science Guide).
• This PR does not modify the documentation, and no documentation update is needed.
• This PR does not modify the documentation, but a documentation update is needed and tracked in issue #_____.

Tier rationale

• Expected tier: 0
• Why: Minor fix

AI assistance disclosure

• No AI tools were used to prepare this PR.
• AI tools were used. Tool(s):  
  
  What was generated (code, tests, documentation, commit messages):
  
  I have reviewed the generated content and take responsibility for it.

Checklist

• This PR closes exactly one tracking issue, linked above.
• The scope of the diff matches the approved scope in the linked issue. No drift, no extras.
• A breaking change is explicitly flagged in the release note sentence above (if applicable).
• The reviewer assigned has the relevant domain knowledge for this change.