Be careful about losing the value in the preptime when just changing the nullability.
Now ConstantExpression can use the inherited field properly.
And when that's null (never) is clearer.
merge MMcM: Fix constant preptime value

Merge pull request #1096 from MMcM/sql-layer/sql-constant-preptime-value
https://github.com/FoundationDB/sql-layer/pull/1096