#5730 Fix error when sending blank signed message

[martgil]

This PR improves the case where sending an empty signed message using FlowCrypt returns unhandled error. This is due to createCleartextMessage() not accepting empty message. So passing a newline that would serve as empty message would satify the condition when user choose to send blank signed email when asked by the FlowCrypt browser extension.

close #5730

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Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• [x] addresses the issue it closes (if any)
• [x] code is readable and understandable
• [x] is accompanied with tests, or tests are not needed
• [x] is free of vulnerabilities
• [x] is documented clearly and usefully, or doesn't need documentation

[martgil]

Hi @sosnovsky, I think this particular one does not need to test for such a small changes. Would you agree on that or should I make another test for sending blank signed message?