Guard node.setSizeMode() against null values in "famous:core:node"

[pilwon]

There is a case where a behavior sends an event to node (ex: size-absolute-x) with $payload being either actual size value or null depends on repeat $index as follows:

{
  '$repeat': [...],
  'size-absolute-x': function($index) {
    return ($index % 2 === 0) ? null : 10;
  }
}

This PR adds null guards to all $public events within famous:core:node such that internally it does not update node.setSizeMode with incorrect value when $payload === null.

[pilwon]

@matthewtoast That was a very quick merge. Thanks! 👍