Handle blank proxy environment variables

[ttarann]

&ElixirMake.Downloader.Httpc.download/1 will fail if HTTP(S)_PROXY environment variables are set blank.

Blank proxy environment variables should be handled as if they are not set.

[whatyouhide]

Hi @ttarann thanks for the report. Blank as in ""?

[ttarann]

@whatyouhide yes, basically "" or all whitespace

[whatyouhide]

I don't think we should ignore all whitespace but we should likely ignore "", yes. Want to send up a PR? 🙃