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

[X] Bug fix

This is a small bug fix, but it can produce more warnings.

Tell us about your environment

• ESLint Version: 6.1.0
• Node Version: 10.16.0
• npm Version: 6.9.0

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

default

Please show your full configuration:

module.exports = {
  parserOptions: {
    ecmaVersion: 2015,
  },
};

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

/*eslint no-dupe-keys: "error"*/

const obj = {
  "": 1,
  "": 2
}

What did you expect to happen?

1 error

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

No errors.

What changes did you make? (Give an overview)

getStaticPropertyName() return value is now explicitly compared to null, because it can be a valid empty string value.

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