While abrensch/brouter@82fecf9 fixed the export already by avoiding the
problematic `OFFR` voice hint, we would still throw in case we
encountered an `undefined` symbol entry:
> Uncaught TypeError: Cannot read properties of undefined (reading
> 'toLowerCase')
This can be prevented by not calling `toLowerCase()` on `undefined`
objects.
Fixes#751
Test Plan:
- Change `VoiceHints.commands` to contain `undefined` entries for a
particular voice hint.
- Create a route with that voice hint.
- Test that exporting for each `turnInstructionMode` does not throw.
8809fa94cf