fix(eslint-plugin): [method-signature-style] fix crash with methods without a return type

[bradzacher]

Fixes #2834

[typescript-eslint]

Thanks for the PR, @bradzacher!

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!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint. As a thank you, your profile/company logo will be added to our main README which receives thousands of unique visitors per day.

[codecov]

Codecov Report

Merging #2836 (92b251d) into master (af9ab87) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2836   +/-   ##
=======================================
  Coverage   92.72%   92.72%           
=======================================
  Files         309      309           
  Lines       10330    10330           
  Branches     2926     2926           
=======================================
  Hits         9579     9579           
  Misses        347      347           
  Partials      404      404           

Flag	Coverage Δ
unittest	92.72% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../eslint-plugin/src/rules/method-signature-style.ts	87.20% <100.00%> (ø)