abstract_collection: Add first contract

Signed-off-by: Florian Deljarry <[email protected]>

Author: Delja

lib/core/collection/abstract_collection.nit

@@ -130,6 +130,8 @@ interface Collection[E]
 	#
 	#     assert [1,2,3].first                == 1
 	fun first: E
+	is
+		expect(length > 0)
 	do
 		return iterator.item
 	end