fix: use LookupEnv to check if envar is actually set

Fixes #389

model.go

@@ -368,9 +368,9 @@ func (v *Value) Reset() error {
 	v.Target.Set(reflect.Zero(v.Target.Type()))
 	if len(v.Tag.Envs) != 0 {
 		for _, env := range v.Tag.Envs {
-			envar := os.Getenv(env)
+			envar, ok := os.LookupEnv(env)
 			// Parse the first non-empty ENV in the list
-			if envar != "" {
+			if ok {
 				err := v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: envar}), v.Target)
 				if err != nil {
 					return fmt.Errorf("%s (from envar %s=%q)", err, env, envar)