Returns all the intent categories or null if none

Property definitions

android $ Intent :: categories
	# Returns all the intent categories or `null` if none
	fun categories: nullable HashSet[String]
	do
		var string_set = intent.categories
		if string_set.is_empty then return null

		return string_set
	end
lib/android/intent/intent_api10.nit:670,2--677,4