RandomDataGenerator seeding fix

Per https://github.com/photonstorm/phaser/issues/1457 the `sow` function
terminates too early on certain false-y values including 0 (and possibly
"").

This changes that, so that 1) only `undefined` and `null` will terminate
the seed sequence processing (this is to maximize backwards compatibility)
and 2) the `length` of the array is honored.

The documentation also reflects the old (and new/altered) behavior.

This is very minor breaking change; hopefully such is mitigated with
leaving in the null/undefined termination.