Processing of NULL assignment values when typed = TRUE

[jankowtf]

Think about how situations should be handled when typed = TRUE (in setReactiveS3() and NULL is specified as the assignment value.

Possible decisions:

• reject if classes don't match (this probably would rarely make sense)
• allow as NULL is in fact somewhat special and might be used at times to denote that certain things are temporarily disabled or thinks like that (probably makes more sense)