[Fix rubocop#1247] Fix an error for Rails/UnusedIgnoredColumns

Fixes rubocop#1247.

This PR fixes an error for `Rails/UnusedIgnoredColumns`
when without tables in db/schema.rb.

changelog/fix_an_error_for_rails_unused_ignored_column.md

@@ -0,0 +1 @@
+* [#1247](https://github.com/rubocop/rubocop-rails/issues/1247): Fix an error for `Rails/UnusedIgnoredColumns` when without tables in db/schema.rb. ([@koic][])

lib/rubocop/rails/schema_loader/schema.rb

@@ -30,6 +30,7 @@ def add_indices_by(table_name:)
 
         def build!(ast)
           raise "Unexpected type: #{ast.type}" unless ast.block_type?
+          return unless ast.body
 
           each_table(ast) do |table_def|
             next unless table_def.method?(:create_table)

spec/rubocop/cop/rails/unused_ignored_columns_spec.rb

@@ -121,6 +121,25 @@ class User < ApplicationRecord
   context 'with no tables db/schema.rb' do
     include_context 'with SchemaLoader'
 
+    let(:schema) { <<~RUBY }
+      ActiveRecord::Schema.define(version: 2020_02_02_075409) do
+      end
+    RUBY
+
+    context 'with an unused ignored column as a Symbol' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          class User < ApplicationRecord
+            self.ignored_columns = [:real_name]
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'with `enable_extension` and no tables db/schema.rb' do
+    include_context 'with SchemaLoader'
+
     let(:schema) { <<~RUBY }
       ActiveRecord::Schema.define(version: 2020_02_02_075409) do
         enable_extension 'plpgsql'