Source property is always wrong

[aylusltd]

Example:

Input:
https://github.com/nexB/gemfileparser2/blob/main/tests/data/gemfiles/Gemfile_1#L94
source "https://rails-assets.org"

Output:
https://github.com/nexB/gemfileparser2/blob/main/tests/data/gemfiles/Gemfile_1-expected.json#L442

{
      "name": "rails-assets-jquery",
      "requirement": [
        "1.11.2"
      ],
      "autorequire": "",
      "source": "",
      "parent": [
        ""
      ],
      "group": "runtime"
    }

Even when not in a source block, source should use the first source in the file.