typesafe format for numbers

interface/src/project/Devices.tsx

@@ -640,6 +640,7 @@ const Devices: FC = () => {
               {me.admin && (
                 <IconButton onClick={customize}>
                   <FormatListNumberedIcon
+                    color="primary"
                     sx={{ fontSize: 18, verticalAlign: 'middle' }}
                   />
                 </IconButton>

interface/src/project/deviceValue.ts

@@ -31,7 +31,7 @@ export function formatValue(
   uom: DeviceValueUOM
 ) {
   if (typeof value !== 'number') {
-    return value;
+    return (value === undefined ? '' : value) as string;
   }
   switch (uom) {
     case DeviceValueUOM.HOURS: