fix: skip fields without env key in onSet (#266)

Options.OnSet gets called on _all_ fields, even the ones that _don't_
have an `env` key defined. This simply ignore those.

Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

env.go

@@ -307,7 +307,9 @@ func get(field reflect.StructField, opts Options) (val string, err error) {
 	}
 
 	if opts.OnSet != nil {
-		opts.OnSet(key, val, isDefault)
+		if ownKey != "" {
+			opts.OnSet(key, val, isDefault)
+		}
 	}
 	return val, err
 }