[Fix #225] Fix an error for Minitest/TestFileName

Fixes #225.

This PR fixes an error for `Minitest/TestFileName` when using empty file.

changelog/fix_an_error_for_minitest_test_file_name.md

@@ -0,0 +1 @@
+* [#225](https://github.com/rubocop/rubocop-minitest/issues/225): Fix an error for `Minitest/TestFileName` when using empty file. ([@koic][])

lib/rubocop/cop/minitest/test_file_name.rb

@@ -21,7 +21,8 @@ class TestFileName < Base
         MSG = 'Test file path should start with `test_` or end with `_test.rb`.'
 
         def on_new_investigation
-          return unless test_file?(processed_source.ast)
+          return unless (ast = processed_source.ast)
+          return unless test_file?(ast)
 
           add_global_offense(MSG) unless valid_file_name?
         end

test/rubocop/cop/minitest/test_file_name_test.rb

@@ -44,4 +44,8 @@ def test_does_not_register_offense_for_non_test_classes
       class Foo; end
     RUBY
   end
+
+  def test_does_not_register_offense_for_empty_file
+    assert_no_offenses('', 'lib/foo.rb')
+  end
 end