I usually prepare enough ice cubes for Shabbat and if I run out do not fill the tray. I do not know if making ice cubes is a Milacha which is allowed or not allowed on Shabbat? i have looked up the 39 Milachas and have not found this addressed thank you.
One should not make Ice cubes on Shabbat. In certain cases if you really need to, what we call "Sha'at Hadchak" it can be permitted. (Shmirat Shabbat Ke'hilchatah). (There are other opinions such as the "Tzitz Eliezer" who do permit making ice cubes on Shabbat). Rabbi Binyamin Bamberger