Major Description
emptyElementParsePolicy was a proposed extension to DFDL at the time we implemented it.
However, it was accepted and is part of DFDL v1.0.
Hence, we need to implement it as an official DFDL property, not just as an extension.
Note that our extension dfdlx:emptyElementParsePolicy had enums 'treatAsEmpty' and 'treatAsMissing'.
The official DFDL 1.0 property renamed 'treatAsMissing' to 'treatAsAbsent'.
The meanings of the enums did not change.
We should continue to accept the extension dfdlx property with the original enum values, and detect if both the official and extension properties exist and are in conflict.
We should issue a suppressable warning about the extension property, suggesting upgrading the DFDL schema to use the official property.
The new official emptyElementParsePolicy should, if not defined, generate a suppressable warning, and get a default 'treatAsEmpty'.