fix(typescript-estree): make FunctionDeclaration.body non-null

[G-Rath]

This is half of #734:

I didn't realise it, but in #156, it added handling to convert function declarations with no body into TSFunctionDeclaration (https://github.com/typescript-eslint/typescript-eslint/pull/156/files#diff-68b2b0b0d2d0ff1846b1114472008332R674)

So it looks like there's no case now that a FunctionDeclaration can have no body?

[typescript-eslint]

Thanks for the PR, @G-Rath!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[bradzacher]

LGTM - thanks for doing this!

[G-Rath]

No problem - I've almost finished doing the second half, which I'll push up in a few minutes for review.