Deserialize empty collections from the query string into `None` (#1050)

We were instead providing empty collections e.g. `Some(Vec::new())`,
which makes protocol tests fail.

This commit also passes Smithy member names through the symbol provider
to obtain safe variable names.

This commit also adds some plumbing to the server protocol test case
generation to fix broken request test definitions in the
`awslabs/smithy` project.