fix: handle empty comment body in Jira webhooks

[nevyangelova]

Summary

When Jira sends a webhook with an empty or whitespace only comment body he plugin was still generating a quote prefix resulting in posts that appeared to have content but displayed nothing. sometimes the comment body is posted and sometimes it's not. This modifies quoteIssueComment() to return an empty string when the comment is empty or contains only whitespace instead of returning just the quote.

Ticket Link

https://mattermost.atlassian.net/browse/MM-67203

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Fixed handling of empty or whitespace-only input to prevent unnecessary formatting processing.

• Tests

  • Added comprehensive test coverage for comment processing edge cases, including single-line, multi-line, empty, and whitespace-only inputs.

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✨ Finishing Touches

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[avasconcelos114]

LGTM!

[ogi-m]

Tested on Jira Server. I don't see a way to add a whitespace-only comment on Jira Cloud. the Save button is disabled.

This fixed it for the subscription notifications:

But, the DM notifications from the Jira bot are still showing the spaces:

[nevyangelova]

@ogi-m fixed