Draw the edge sparkline only for a value it can plot

The popover showed a non-numeric value twice: only numbers are recorded,
so MessageSparkline fell through to a collapsed ValuePreview and the
inspector rendered its own expanded one below it.

frontend/src/components/Flow/EdgeInspector.tsx

@@ -132,15 +132,18 @@ export function EdgeInspector({
         </div>
 
         {/* The same curve the node panel draws for this message: one value
-            says little, how it has been moving says the rest. */}
-        {live === undefined ? null : (
+            says little, how it has been moving says the rest. Only for a
+            number, because only numbers are recorded — for anything else the
+            sparkline falls through to its own copy of the value, which the
+            popover already shows below. */}
+        {typeof live?.value === "number" ? (
           <div className="mt-2">
             <MessageSparkline
               flow={flow}
               name={displayName(flow, edge.message)}
             />
           </div>
-        )}
+        ) : null}
 
         {/* Several nodes can publish one message, and so can a dashboard or
             another flow — so the value alone does not say what caused it. */}