fix: handle empty proxy environment variables

[benoitc]

Summary

When proxy environment variables (http_proxy, https_proxy, etc.) are set to empty strings or whitespace-only values, hackney crashes with a pattern match error in hackney_url:parse_netloc/2.

Problem

• export http_proxy="" causes all hackney requests to fail
• The empty string is passed to URL parsing which expects a valid URL format
• This affects users who clear proxy variables by setting them to empty strings

Solution

• Check for empty strings in do_get_proxy_env/1 before attempting to parse
• Strip whitespace from proxy URLs and treat whitespace-only values as empty
• Skip to the next proxy variable in the list when empty values are found

Test plan

• Added comprehensive unit tests in test/hackney_proxy_tests.erl
• Tests cover empty strings, whitespace-only strings, and valid proxy URLs
• Integration tests verify that requests work correctly with empty proxy variables
• Exported internal functions for testing when compiled with -DTEST flag