Fix calling `clear` on nil when transaction is rolled back

I have no idea why it fails for me. But might be something about nested transactions in rspec:
```
Caught NoMethodError: undefined method `clear' for nil:NilClass
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/after_commit_queue-1.0.1/lib/after_commit_queue.rb:34:in `_clear_after_commit_queue'
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `_run__2618979648383371027__rollback__485157533732720751__callbacks'
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:405:in `__run_callback'
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:385:in `_run_rollback_callbacks'
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/activesupport-3.2.13/lib/active_support/callbacks.rb:81:in `run_callbacks'
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/activerecord-3.2.13/lib/active_record/transactions.rb:289:in `rolledback!'
	/Users/mikz/.rvm/gems/ruby-1.9.3-p374-railsexpress@3scale3/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/database_statements.rb:357:in `block in rollback_transaction_records'
```

So I've made patch to always initialize the array before clearing it. It prevents all calls on nil.