Overriding REZ_CONTEXT_TRACKING_HOST in a shell results in an error for every resolve

When setting context_tracking_host to an empty string in rezconfig.py everything works as expected. If the setting is set to a host but overriden using an empty environment variable (`set "REZ_CONTEXT_TRACKING_HOST= "`) every resolve yields an error:

`11:53:00 ERROR Cannot connect to the message broker: [Errno 11004] getaddrinfo failed`

Not sure if this is windows specific

Reproduce a resolve with context_tracking_host set in rezconfig.py and then overridden by the empty REZ_CONTEXT_TRACKING_HOST environment variable. Trace the environment-setting handling and resolve path; done means the override behaves like an empty configuration value without producing the message-broker getaddrinfo error.

Written by the indexing model from the issue text.