Add if-empty() as an alias of once()

From https://forum.opendatakit.org/t/copy-a-field-into-another/6808/

> I wonder if using once() would meet your requirements. The name of this function is not very descriptive as it works like if_empty() and therefore wouldn't override a user-entered value, unless the user-entered value is empty. We should perhaps consider adding an alias for once().

Locate the specification entry for once() and review how function aliases are defined. Add if-empty() as its alias, ensuring the specification clearly records that both names have the same behavior; the issue names no file or test, so completion is a consistent specification update.

Written by the indexing model from the issue text.