Support Unit values in Query return types.

- Emit a dummy value ("select 1 from ...") in
  JdbcStatementBuilderComponent for a query which returns an empty
  ProductNode or StructNode.

- Add a UnitResultConverter of width 0 and create this instead of a
  ProductResultConverter for empty ProductNodes.

- Return classTag[Unit] for arity 0 in TupleSupport.classTagForArity.

Tests in RelationalTypeTest.testUnit. Fixes issue #742.
Merge pull request #793 from slick/tmp/issue-742

Support Unit values in Query return types.