Fix KeyError when shared key has no adapter in database.yml

Follow-up #1605 (comment)

Author: koic

lib/rubocop/cop/mixin/database_type_resolvable.rb

@@ -59,7 +59,7 @@ def database_yaml(environment = 'development')
       end
 
       def database_adapter
-        database_yaml['adapter'] || database_yaml('shared')&.fetch('adapter') || database_yaml.first.last['adapter']
+        database_yaml['adapter'] || database_yaml('shared')&.dig('adapter') || database_yaml.first.last['adapter']
       end
     end
   end

spec/rubocop/cop/rails/bulk_change_table_spec.rb

@@ -708,6 +708,19 @@ def change
           it_behaves_like 'no offense for postgresql'
         end
       end
+
+      context 'without adapter configuration' do
+        let(:yaml) do
+          {
+            'shared' => {},
+            'development' => {
+              'foo' => 'bar'
+            }
+          }
+        end
+
+        it_behaves_like 'no offense'
+      end
     end
 
     context 'invalid (e.g. ERB)' do