Ref T2222. Five very small improvements:

• I hit this exception and it took a bit to understand which transaction was causing problems. Add an Exception subclass which does a better job of making the message debuggable.
• The oldValue of a transaction may be null, legitimately (for example, changing the repositoryPHID for a revision from null to some valid PHID). Do a check to see if setOldValue() has been called, instead of a check for a null value.
• Add an additional check for the other case (shouldn't have a value, but does).
• When we're not generating a value, don't bother calling the code to generate it. The best case scenario is that it has no effect; any effect it might have (changing the value) is always wrong.
• Maniphest didn't fall back to the parent correctly when computing this flag, so it got the wrong result for CustomField transactions.

Resolved the issue I was hitting more easily, made updates to a null-valued custom field, and applied other normal sorts of transactions successfully.