Use default gemfile path instead of Rails root when listing engines

[vinistock]

Motivation

For projects that aren't Rails, such as Tapioca itself, Rails.root is nil, which then breaks when trying to invoke Rails.root.to_path.

Implementation

Add a check for verifying that Rails.root is set before continuing down to list engines. This enables Tapioca to generate RBIs for itself again.

Tests

I couldn't figure out a way of forcing Rails.root to return nil. Let me know if you have ideas.

[paracycle]

I don't think this is the correct way to go about this, since the Rails.root is not even really needed except for filtering app engines.

I'd rather that we do the same thing we do for filtering out app gems, which is to use the folder of the Gemfile instead of something like Rails.root. Ref:

tapioca/lib/tapioca/gemfile.rb

Lines 131 to 133 in 39b8a4b

	def dir
	File.expand_path(gemfile.path + "/..")
	end

Didn't see Ufuk's comment when submitted

[paracycle]

Basically I'd rather we use Bundler.default_gemfile.parent.expand_path instead of Rails.root.to_path when doing the filtering.