Fix decorated_parameters for when method has no parameters

[andyw8]

Motivation

When indexing a method with no arguments, it was failing with:

NoMethodError: private method `format' called for an instance of RubyIndexer::Entry::Signature

(because signatures.first was returning nil, and format is defined on nil as a private method)

Implementation

Hand the case of a method with no arguments.

Automated Tests

Updated

Manual Tests

Add a file with a method with no parameters. Save it and ensure there are no errors in the console.

[andyw8]

Actually, this may be a little more involved.

On this branch, if you open a new file and type class, there's no longer and error but you'll see this:

Since class is very often the first thing someone will type in a file, it's not a great experience.

[vinistock]

I understand the concern, but I will say that those are separate in my opinion. The behaviour is correct: you can invoke class on the top level because it is an instance of Object. What we're missing is completion for keywords, so that you'd get a second element for it.

If you prefer to wait shipping that's fine, but I'm not opposed to moving ahead and handling keywords later.