env tag option emptyOverridesDefault

[shestakovich]

emptyOverridesDefault: treats an empty environment variable value as an explicit override of the default value

func TestEmptyOverridesDefault(t *testing.T) {
	type Test struct {
		Foo    string `env:"FOO" envDefault:"foo"`
		Bar    string `env:"BAR,emptyOverridesDefault" envDefault:"bar"`
		NotSet string `env:"NOT_SET,emptyOverridesDefault" envDefault:"default"`
	}

	t.Setenv("FOO", "")
	t.Setenv("BAR", "")

	var cfg Test
	isNoErr(t, Parse(&cfg))
	isEqual(t, "foo", cfg.Foo)
	isEqual(t, "", cfg.Bar)
	isEqual(t, "default", cfg.NotSet)
}