Initial values setup resulted in label values (eg "+2") to be numbers (ie 2), and was changing to strings by iron-select upon user changing label value. With this change, gr-label-score-row.selectedValue returns string or null if not selected. This is needed for consistency of the plugin API. Change-Id: Ia1ca49de498a9a0552740c6ad1aad8acd21c4f3b