[Fix rubocop#997] Support methods and variables in NotNullColumn

At the NotNullColumn cop if a method or variable is passed as arguments for `add_column`
an error is triggerred:
```
NoMethodError:
  undefined method `value' for s(:send, nil, :string):RuboCop::AST::SendNode
```

In this commit we an extra check to ensure the type has  the
`value` method implemented, before invoking it.

changelog/fix_not_allow_not_null_to_work_with_methods_and_variables.md

@@ -0,0 +1 @@
+* [#997](https://github.com/rubocop/rubocop/issues/997): Fix to Allow `NotNullColumn` to work with method calls and variables. ([@fidalgo][])

lib/rubocop/cop/rails/not_null_column.rb

@@ -45,7 +45,7 @@ def on_send(node)
 
         def check_add_column(node)
           add_not_null_column?(node) do |type, pairs|
-            return if type.value == :virtual || type.value == 'virtual'
+            return if type.respond_to?(:value) && (type.value == :virtual || type.value == 'virtual')
 
             check_pairs(pairs)
           end

spec/rubocop/cop/rails/not_null_column_spec.rb

@@ -21,6 +21,14 @@
       end
     end
 
+    context 'with the type argument is a variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          add_column(:users, :name, type, default: 'default')
+        RUBY
+      end
+    end
+
     context 'with null: false and default: nil' do
       it 'reports an offense' do
         expect_offense(<<~RUBY)