jams-react-client: fix default color when enabling background

At the moment, when you enable a custom background for a blueprint, it
generates an invalid JSON where the color is undefined

Change-Id: I5a45b1b55fa12b5803326370627fa74cf96fd9aa

jams-react-client/src/views/Blueprint/updatePolicyData.tsx

@@ -112,7 +112,7 @@ const updateUiCustomization = (data) => {
     ui.backgroundColorOrUrl = backgroundUrl;
   } else if (backgroundColor !== "") {
     ui.backgroundType = "color";
-    ui.backgroundColorOrUrl = backgroundColor;
+    ui.backgroundColorOrUrl = backgroundColor || "#ffffff";
   } else {
     ui.backgroundType = "default";
   }