atomic argument ignored when document argument is null

[ehennum]

When a Data Service supports both atomic and document / node parameters
and the document / node parameter is nullable
and the document / node argument is null
then the atomic argument is ignored.

[ehennum]

All the unit tests ran green, so this tweak should cause no regressions.

The work in progress on #1296 has a unit test for this edge case, so a separate test now is probably unnecessary. Any concerns, @georgeajit ?

Copying you, @llinggit , for a sanity check on the 2ff0a5e commit. (A formal code review on a pull request in a separate branch seemed like overkill for this one line change.)