Correctly handle emtpy string in proxyuserpwd config

As documented, the default value for config value proxyuserpwd is ''.
However, that value results in the error:
 "cURL error 5: Unsupported proxy syntax in '@'".
This patch handles the values of '' and null (the default in the code)
the same for config values proxyuserpwd and proxy.

Signed-off-by: Scott Shambarger <devel@shambarger.net>