We currently don't have a function for that.
I have added it as an Idea to Ideas board
https://powerusers.microsoft.com/t5/PowerApps-Ideas/Function-to-produce-blank-value/idi-p/14353#M964
Please upvote on this so the team can prioritize this.
For now, I can think of a workaround using our current language syntax: unspecificed value in a If condition is treated as NULL.
If you could change your expression to the following, it will work:
WOSAPI.WOSV4PatchByWONUM("111", {COMPLETEDATE: If(Text(DatePicker3.SelectedDate, DateTimeFormat.UTC) <> "", Text(DatePicker3.SelectedDate, DateTimeFormat.UTC))})
meaning if the value is empty string, no value i.e. NULL is specified.
Hope this helps
Thanks
Murali