fix: ignore empty proxy_env

[feng19]

When all_proxy= it will throw an error:

[error] GenServer #PID<0.2401.0> terminating
** (CaseClauseError) no case clause matching: []
    (hackney 1.20.1) deps/hackney/src/hackney_url.erl:248: :hackney_url.parse_netloc/2
    (hackney 1.20.1) deps/hackney/src/hackney.erl:691: :hackney.proxy_from_url/5
    (hackney 1.20.1) deps/hackney/src/hackney.erl:335: :hackney.request/5
    (tesla 1.8.0) lib/tesla/adapter/hackney.ex:75: Tesla.Adapter.Hackney.request/5
    (tesla 1.8.0) lib/tesla/adapter/hackney.ex:37: Tesla.Adapter.Hackney.call/2

[benoitc]

this should be merged in #737 . Working on it. Thanks for the patch