switch N/A for blank

app/src/frontend/building/data-containers/age.tsx

@@ -18,7 +18,7 @@ const AgeView: React.FunctionComponent<CategoryViewProps> = (props) => {
     const currentYear = new Date().getFullYear();
     if (props.building.date_source == "Expert knowledge of building" ||
         props.building.date_source == "Expert estimate from image" ||
-        props.building.date_source == "N/A"
+        props.building.date_source == ""
        ){
       return (
           <Fragment>

app/src/frontend/config/data-fields-config.ts

@@ -285,7 +285,7 @@ export const dataFields = { /* eslint-disable @typescript-eslint/camelcase */
             "Other website",
             "Other"
         ],
-        example: "N/A",
+        example: "",
     },
     date_source_detail: {
         category: Category.Age,