Only supply units if they were specified, to avoid seeing "units: null" in so many properties, see https://github.com/phetsims/phet-io/issues/1315
Only supply units if they were specified, to avoid seeing "units: null" in so many properties, see https://github.com/phetsims/phet-io/issues/1315

phetsims/axon,phetsims/axon,phetsims/axon