prevent downstream nil exception when view_definition is nil.

Fixes #330. it looks like there is more than one way to collect column names, but I'm not sure what 'real' column names are in the context of [schema_statements.rb#L253](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/blob/master/lib/active_record/connection_adapters/sqlserver/schema_statements.rb#L253).  Is my fix ok, or does it mask other issues?