Fix extract_backtraces migration, account for notices without backtrace

I got the following exception when migrating:

    Migrating to ExtractBacktraces (20121003223358)
    ==  ExtractBacktraces: migrating ==============================================
    -- It could take long time (hours if you have many Notices)
    rake aborted!
    An error has occurred, 20121003223358 and all later migrations canceled:

    undefined method `each' for nil:NilClass
    /app/app/models/backtrace.rb:26:in `raw='
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/attributes/processing.rb:98:in `process_attribute'
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/attributes/processing.rb:26:in `block in process'
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/attributes/processing.rb:24:in `each_pair'
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/attributes/processing.rb:24:in `process'
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/document.rb:136:in `block in initialize'
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/threaded/lifecycle.rb:84:in `_building'
    /app/vendor/bundle/ruby/1.9.1/gems/mongoid-2.4.10/lib/mongoid/document.rb:130:in `initialize'
    /app/app/models/backtrace.rb:18:in `new'
    /app/app/models/backtrace.rb:18:in `find_or_create'
    /app/db/migrate/20121003223358_extract_backtraces.rb:5:in `block in up'

Passing an empty array instead of `nil` as the `raw` argument fixed this.