Fix for query parameter serialization with empty string values

[pactender]

Hi maintainers,

I have prepared a minimal fix and regression tests for handling query parameter tuples with explicit empty string values:
https://github.com/pactender/httpx/tree/fix/query-param-empty-values

Since direct pull requests are currently restricted to collaborators on this repository, please take a look at the commit (pactender/httpx@849701c) when convenient, or let me know if a collaborator can pull this in. Thanks!