[Fix #979] Fix an error for Rails/ThreeStateBooleanColumn

Fixes #979.

This PR fixes an error for `Rails/ThreeStateBooleanColumn`
when using `t.boolean` in `drop_table`.

Author: koic

changelog/fix_an_error_for_rails_three_state_boolean.md

@@ -0,0 +1 @@
+* [#979](https://github.com/rubocop/rubocop-rails/issues/979): Fix an error for `Rails/ThreeStateBooleanColumn` when using `t.boolean` in `drop_table`. ([@koic][])

lib/rubocop/cop/rails/three_state_boolean_column.rb

@@ -46,7 +46,9 @@ def on_send(node)
 
             def_node = node.each_ancestor(:def, :defs).first
             table_node = table_node(node)
-            return if def_node && change_column_null?(def_node, table_node.value, column_node.value)
+            if def_node && (table_node.nil? || change_column_null?(def_node, table_node.value, column_node.value))
+              return
+            end
 
             add_offense(node)
           end

spec/rubocop/cop/rails/three_state_boolean_column_spec.rb

@@ -115,6 +115,16 @@ def change
       RUBY
     end
 
+    it 'does not register an offense when using `t.boolean` in `drop_table`' do
+      expect_no_offenses(<<~RUBY)
+        def change
+          drop_table(:users) do |t|
+            t.boolean :active
+          end
+        end
+      RUBY
+    end
+
     it 'does not register an offense when using `#change_column_null`' do
       expect_no_offenses(<<~RUBY)
         def change