Skip defaults on non zero values

[davidterranova]

The goal of this PR is to apply defaults only if the two following conditions are matched :

• No environment variable matches the struct field
• The struct field value is the zero value

This change allow envconfig to be integrated with a more general config system that loads the config from a different provider and override values from environment variables if they are defined / with a zero value

[draev]

Up! It's a useful fix!