URI builder starts query strings with ampersand

**Describe the bug**
We encountered a bug in DDN Infinia where list requests were failing. We tracked it down to an unusual URI in the format of `http://server/bucket?&list-type=2&prefix=foo/`. Starting a query string with an ampersand (before `list-type`) is unusual and broke our URI parser.

**To Reproduce**
Run a list objects request with the AWS plugin and trace the URI generated.

**Expected behavior**
The URI should not start a query string with an ampersand. The above URI should be:
`http://server/bucket?list-type=2&prefix=foo/`

**Additional context**
We have fixed our parser to handle this corner case, but have customers we cannot upgrade right now so we would like to fix it in the crate.

I have a PR I will submit shortly.

Start by running a list objects request through the AWS plugin and tracing where its URI is assembled. Inspect the generated query delimiter before list-type and add coverage for this case; done means the URI begins with ?list-type=2&prefix=foo/ rather than ?&list-type=2&prefix=foo/.

Written by the indexing model from the issue text.