[SSR] `className` reflection should handle non-strings

[nolanlawson]

Looking at our implementation of className in the SSR runtime, it seems we have a small bug, which is that we don't coerce to a string when you set it to a non-string, e.g.:

this.className = undefined // should be 'undefined'
this.className = null // should be 'null'
this.className = 0 // should be '0'
// etc.

This matches what browsers do in this case. Not sure about engine-server.