As per this discussion:
Currently, the only way to make a custom event argument optional is by inputting a default value. This default value cannot be an empty string, otherwise the argument will be mandatory.
I (and 2 other developers in the linked thread) have run into situations where it would be desirable to make an argument optional, but take on a value of null (or empty string) as the default value.
Would be nice to have a "null default value" checkbox or similar functionality in the custom event dialog.
Any word on this? It's pretty standard ;D following up after a year