PR# 15501 `index_set' fails for an empty array

Feature ARRAY.index_set has a postcondition (Result.lower = lower) and (Result.upper = upper). In an empty array lower = x and upper = x - 1, where x can be any integer. However, when calling `index_set' on an empty array, the resulting integer interval always has lower = 1 and upper = 0, which violates the postcondition.

	execute is
			-- Run application.
		local
			a: ARRAY [INTEGER]
			i: INTEGER_INTERVAL
		do
			create a.make (10, 9)
			i := a.index_set
		end