Change TextStyle nullability to non-null

[joecks]

It is contra intuitive to expose the Textstyle in the XStyles class as nullable ? . All textsyles are depended on the use of Theme.headline1 which it self can be nullable, but as a user of Linden I expect that Theme.headline is set, otherwise I would expect a message from the system telling me to set it (like a crash - assertNotNull)