Fix: space-in-parens crash

[mysticatea]

What is the purpose of this pull request? (put an "X" next to item)

[X] Bug fix (template)

Tell us about your environment

• ESLint Version: master
• Node Version: 8.9.1
• npm Version: 5.5.1

What parser (default, Babel-ESLint, etc.) are you using?

• default

Please show your full configuration:

• nothing

What did you do? Please include the actual source code causing the issue.

/* eslint space-in-parens: [error, always, {}] */
1

I couldn't get the URL of our online demo because it crashes. Please copy/paste then see devtools' console.

What did you expect to happen?

No errors.

What actually happened? Please include the actual, raw output from ESLint.

A type error was thrown.

Uncaught TypeError: Error while loading rule 'space-in-parens': Cannot read property 'length' of undefined

What changes did you make? (Give an overview)

This PR fixes space-in-parens rule not crashing.

Is there anything you'd like reviewers to focus on?

Nothing in particular.

[platinumazure]

LGTM, thanks!