VirtualMachine getTags does not return Tags with empty values

[plombardi89]

AWS allows an Instance to have a Tag with an empty value. I created such a tag and Dasein does not return the tag. I then modified the tag so it had a value and Dasein returned it perfectly fine.

Notice the purpose_foo_master

Without tag now that value of tag has been removed in AWS:

This using Dasein 2013.07.02