fix(messages): reject blank encrypted payloads

[aiirvizionz]

Summary

• reject blank encryptedContents values before send-message database work
• prevent whitespace-only encrypted payloads from being base64-encoded into recipient copies
• add regression coverage for blank encrypted content strings

Test

• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs "src/app/api/messages/send/route.test.js"
• git diff --check

uGig billing

• Expected invoice: 0.50 USD if accepted/merged
• Counted as revenue now: no, pending review/payment