Rails 4.2: Set @nchar and @object_type only when sql_type is true

This pull request temporary addresses following error by checking `sql_type` exists.

```ruby
$ rake test_oracle
... snip ...
/home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_column.rb:29:in `initialize': undefined method `[]' for nil:NilClass (NoMethodError)
        from /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:1201:in `new'
        from /home/yahonda/git/oracle-enhanced/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb:1201:in `new_column'
        from /home/yahonda/git/rails/activerecord/lib/active_record/attributes.rb:80:in `attribute'
        from /home/yahonda/git/rails/activerecord/test/cases/attributes_test.rb:5:in `<class:OverloadedType>'
        from /home/yahonda/git/rails/activerecord/test/cases/attributes_test.rb:3:in `<top (required)>'
        from /home/yahonda/git/rails/activesupport/lib/active_support/dependencies.rb:248:in `require'
        from /home/yahonda/git/rails/activesupport/lib/active_support/dependencies.rb:248:in `block in require'
        from /home/yahonda/git/rails/activesupport/lib/active_support/dependencies.rb:233:in `load_dependency'
        from /home/yahonda/git/rails/activesupport/lib/active_support/dependencies.rb:248:in `require'
        from /home/yahonda/.rvm/gems/ruby-2.1.2@rails42/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:15:in `block in <main>'
        from /home/yahonda/.rvm/gems/ruby-2.1.2@rails42/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `select'
        from /home/yahonda/.rvm/gems/ruby-2.1.2@rails42/gems/rake-10.3.2/lib/rake/rake_test_loader.rb:4:in `<main>'
rake aborted!
```