Warn on and ignore null or undefined values

[paulwehner]

I made a stackoverflow outlining the issue:

http://stackoverflow.com/questions/31264376/reactjs-radium-altjs-uncaught-typeerror-cannot-read-property-tostring-of

[ianobermiller]

The only time prefixer calls toString is on the value of a property when it is not a string. So if you are seeing this error, you must be passing null or undefined as the value for some property. Radium should probably just warn you about this and ignore it.