Fix listing blobs when there are no blobs in a container

[carols10cents]

Hi, I was trying out the code added in #176, and if a container doesn't have any blobs in it, serde can't deserialize it. If I run the list_blobs_01 example as it is in my first commit, it panics with this error:

Error: SerdeXMLDeserializationError(Custom { field: "missing field `Blob`" })

The second commit in this PR fixes it by setting a default of an empty vector on the blobs field.

[ghost]

All CLA requirements met.

[heaths]

Thank you! I like this change. In the official Azure SDKs, we try to avoid returning null collections, so returning a new empty collection works well.

[MindFlavor]

Thank you, I also prefer an empty vector instead of an option in this case!

We should probably add a specific test in the E2E test collection to catch these bugs early.