We added this for String some time ago, so let's give Utf16String the
same optimization. Note that Utf16String was already handling its data
member potentially being null as of 5af99f4dd0.
Utf16FlyString more or less works exactly the same as FlyString. It will
store the raw encoded data of the string instance. If the string is a
short ASCII string, Utf16FlyString holds the ShortString bytes; else,
Utf16FlyString holds a pointer to the Utf16StringData.