Handle ignored_columns from mixins

[tachyons]

Ignored columns can be added from a rails concern. In such cases, Rubocop fails with error as fetching table logic fails.

Before

Failure/Error:
       def_node_search :find_belongs_to, <<~PATTERN
         (send nil? :belongs_to {str sym} ...)
       PATTERN

     NoMethodError:
       undefined method `each_node' for nil:NilClass

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[pirj]

Thanks!
Would you like to add a changelog entry similar to this (all are removed during the release procedure) one? 3cae05d#diff-84b0e0ad32ba865c4f40649f0e502782883e4f0d90767d50e13d684bf9674c0d

[tachyons]

@pirj Just added one, Can you check if that is proper format ?

[koic]

Thanks!