Return early if table name is nil

When trying to upgrade to version 1.3.0, I encountered an error when running the MSSQL adapter that previously wasn't there on 1.2.9.1. I'm using [PaperTrail](https://github.com/airblade/paper_trail) and when calling the `has_paper_trail` method from an abstract model, one of its callbacks was sending `nil` to the connection adapter's `table_exists?` method causing an `Argument Error: nil table name` to be raised.

It could be argued that the caller shouldn't pass `nil`, but I noticed that Rails' adapters check for the `nil` case (e.g. [mysql adapter](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb#L429)).