Stylistic comment: would probably be easier to read if builder was declared+used in-line inside the if block to return the result. It's not needed outside. Then there could be an else (or fall-through) to return `MutableMap.of()`. (ideally would want to support returning `ImmutableMap.of()`, but don't trust super + sub-classes to not try to modify the returned map!)

No strong feelings; feel free to ignore.