`aws-inventory rds` fails due to comparison of NilClass with String

Here's the stack trace:

```
C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/lib/aws_inventory/base.rb:23:in `sort_by': comparison of NilClass with String failed (ArgumentError)
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/lib/aws_inventory/base.rb:23:in `sort'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/lib/aws_inventory/base.rb:16:in `report'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/lib/aws_inventory/rds.rb:8:in `report'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/lib/aws_inventory/cli.rb:35:in `rds'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/thor-0.20.0/lib/thor/command.rb:27:in `run'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/thor-0.20.0/lib/thor/invocation.rb:126:in `invoke_command'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/thor-0.20.0/lib/thor.rb:387:in `dispatch'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/lib/aws_inventory/command.rb:21:in `dispatch'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/thor-0.20.0/lib/thor/base.rb:466:in `start'
from C:/Ruby24-x64/lib/ruby/gems/2.4.0/gems/aws-inventory-0.4.3/exe/aws-inventory:14:in `'
from C:/Ruby24-x64/bin/aws-inventory:23:in `load'
from C:/Ruby24-x64/bin/aws-inventory:23:in `'
```