Exception on global Rubocop errors

[daniel-rikowski]

Whenever erb_lint tries to record a rubocop offense which affects a whole file itself, e.g. Naming/FileName or Lint/EmptyFile,
then ERBLint::ProcessedSource#to_source_range causes a runtime error "undefined method 'begin' for an instance of RuboCop::Cop::Offense::PseudoSourceRange"

This happens because the range argument in such offenses is a RuboCop::Cop::Offense::PseudoSourceRange without a begin or end method.

This is also the root cause for #388

Rubocop background:

• https://github.com/rubocop/rubocop/blob/master/lib/rubocop/cop/offense.rb#L80
• https://github.com/rubocop/rubocop/blob/master/lib/rubocop/cop/base.rb#L189-L195