Make a Save operation to a table with only an Id field always work.

The save operation did not work when the record is already present because the
generated update SQL is invalid, it has no "set" arguments. Admittedly such an
operation is useless, but generating bad SQL is worse, so make the save silently
do nothing except for a debug log entry.
Remove extraneous variable added by mistake and add some update tests.
Merge pull request #363 from BruceCowan-AI/FixNullUpdateToSilentlyDoNothing

Make a Save operation to a table with only an Id field always work.