client/web: fix nil metricCapture crash in mockLocalAPI

The upload-client-metrics handler called metricCapture without
checking if it was nil or if the metrics slice was empty. Most
tests pass nil for metricCapture, so if a metrics upload races
in during the test, it panics.

Fixes #19252

Change-Id: Ib904d1fe6779067dc2a153d1680b8f50cba9c773
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/web/web_test.go

@@ -1477,7 +1477,9 @@ func mockLocalAPI(t *testing.T, whoIs map[string]*apitype.WhoIsResponse, self fu
 				http.Error(w, "invalid JSON body", http.StatusBadRequest)
 				return
 			}
-			metricCapture(metricNames[0].Name)
+			if metricCapture != nil && len(metricNames) > 0 {
+				metricCapture(metricNames[0].Name)
+			}
 			writeJSON(w, struct{}{})
 			return
 		case "/localapi/v0/logout":