fix: enforce implicit return assertion for non-empty function bodies

[ncthuc2004]

Summary

• fix implicit return assertion insertion for non-empty function bodies in babel-plugin-flow-runtime
• correct last-statement check so a synthetic return; is added when there is no explicit ReturnStatement
• add regression fixture for a function body with statements and no explicit return

Why

Issue #2 reports missing return assertions for implicit returns.
The current condition uses incorrect boolean precedence and fails to append return; in non-empty bodies unless the last statement is explicitly return.

Validation

• targeted transform test:
  • TEST_FILTER=2-implicit-function-return-with-body
  • npx mocha --opts NUL --require @babel/polyfill --require ./test-polyfill.js src/__tests__/transform.test.js --timeout 10000
• result: 1 passing

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #2: Add assertions when implicitly returning from a function

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