Handle sections without text parameters

Allows interop with gems that throw custom exceptions without string messages.

Without this fix, if a recipe uses a `ruby_block` and the block throws an exception with no explanatory message (for example, the Cassandra gem throws `SchemaDisagreementException`s in some edge conditions), we hit the following exception:

```
Generated at 2013-11-07 07:51:01 +0000
TypeError: can't convert nil into String
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/highline-1.6.15/lib/highline/style.rb:131:in `+'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/highline-1.6.15/lib/highline/style.rb:131:in `color'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/highline-1.6.15/lib/highline.rb:365:in `color'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/highline-1.6.15/lib/highline.rb:380:in `color'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/base.rb:91:in `puts'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/error_descriptor.rb:60:in `display_section'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/error_descriptor.rb:43:in `block in display'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/error_descriptor.rb:42:in `each'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/error_descriptor.rb:42:in `display'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/base.rb:130:in `display_error'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/base.rb:161:in `resource_failed'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/formatters/doc.rb:159:in `resource_failed'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/event_dispatch/dispatcher.rb:29:in `block in resource_failed'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/event_dispatch/dispatcher.rb:29:in `each'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/event_dispatch/dispatcher.rb:29:in `resource_failed'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource.rb:615:in `rescue in run_action'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource.rb:621:in `run_action'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/runner.rb:50:in `run_action'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/runner.rb:82:in `block (2 levels) in converge'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/runner.rb:82:in `each'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/runner.rb:82:in `block in converge'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection.rb:94:in `block in execute_each_resource'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection/stepable_iterator.rb:116:in `call'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection/stepable_iterator.rb:116:in `call_iterator_block'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection/stepable_iterator.rb:85:in `step'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection/stepable_iterator.rb:104:in `iterate'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection/stepable_iterator.rb:55:in `each_with_index'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/resource_collection.rb:92:in `execute_each_resource'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/runner.rb:81:in `converge'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/client.rb:404:in `converge'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/client.rb:469:in `do_run'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/client.rb:200:in `run'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application.rb:190:in `run_chef_client'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application/client.rb:297:in `block in run_application'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application/client.rb:290:in `loop'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application/client.rb:290:in `run_application'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application.rb:73:in `run'
/opt/chef/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/bin/chef-client:26:in `<top (required)>'
/usr/bin/chef-client:23:in `load'
/usr/bin/chef-client:23:in `<main>'
```

Unfortunately, this is then the exception backtrace that gets printed out by`chef-client`, rather than the original error, making debugging hard.

The proposed fix simply sets the message to `""` if no message was supplied from the exception.