Look at the reverse Do_Not_Emit mappings

[eggrobin]

This would have caught the error fixed in #1246.

[markusicu]

New CI check failure:

TestProperties.TestDefaults:615->TestFmwkMinusMinus.assertNotNull:65
Null Do_Not_Emit_Dispreferred for U+FFFF but property is String with default value type LITERAL.
Add an @missing line to ExtraPropertyValueAliases.txt if needed. ==> expected: not <null>