Fix RSpec/FilePath when checking an empty class

Remember that `RSpec/FilePath` checks *all* files - not only
**/*_spec.rb. And when some class defines e.g. an empty class, the
`TopLevelGroup` module would raise an error.

Fixes #999.

Author: bquorning

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+* Fix `RSpec/FilePath` when checking a file defining e.g. an empty class. ([@bquorning][])
+
 ## 1.43.0 (2020-08-17)
 
 * Add a new base cop class `::RuboCop::Cop::RSpec::Base`. The old base class `::RuboCop::Cop::RSpec::Cop` is deprecated, and will be removed in the next major release. ([@bquorning][])

lib/rubocop/rspec/top_level_group.rb

@@ -38,7 +38,9 @@ def top_level_group?(node)
       end
 
       def top_level_nodes(node)
-        if node.begin_type?
+        if node.nil?
+          []
+        elsif node.begin_type?
           node.children
         elsif node.module_type? || node.class_type?
           top_level_nodes(node.body)

spec/rubocop/cop/rspec/file_path_spec.rb

@@ -204,6 +204,14 @@
     RUBY
   end
 
+  # RSpec/FilePath runs on all files - not only **/*_spec.rb
+  it 'works on files defining an empty class' do
+    expect_no_offenses(<<-RUBY)
+      class Foo
+      end
+    RUBY
+  end
+
   context 'when configured with CustomTransform' do
     let(:cop_config) { { 'CustomTransform' => { 'FooFoo' => 'foofoo' } } }