Special case the empty string in object-literal-key-quotes rule

[gustavderdrache]

PR checklist

• Addresses an existing issue: Rule object-literal-key-quotes mistakenly flags the empty string as not needing quotes #1761
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

What changes did you make?

• The bug in Rule object-literal-key-quotes mistakenly flags the empty string as not needing quotes #1761 appears to be rooted in IDENTIFIER_NAME_REGEX using a * instead of + repeat modifier. This has been addressed.
• I added a test case for '' in the "as-needed" files.

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

I don't know enough about the spirit of "consistent" or "consistent-as-needed" to know if this case should be flagged as a failure or not:

const uncertain = {
  foo: 'bar',
  '': 'baz',
};

Right now, this test fails in both cases. It seems like this is correct, but I welcome confirmation. Once I have it I'll update the tests.

[palantirtech]

Thanks for your interest in palantir/tslint, @gustavderdrache! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[nchen63]

@gustavderdrache thanks!