Allow nil in latest tests

There is a possibility of nil (when we don't have any tag available).
This updates the test to allow such cases and only check if latest is
not nil

managed/services/server/updater_test.go

@@ -272,9 +272,10 @@ func TestUpdater(t *testing.T) {
 		u := NewUpdater(watchtowerURL, gRPCMessageMaxSize)
 		_, latest, err := u.latest(context.Background())
 		require.NoError(t, err)
-		assert.NotNil(t, latest)
-		assert.True(t, strings.HasPrefix(latest.Version.String(), "2.") || strings.HasPrefix(latest.Version.String(), "3."),
-			"latest version of PMM should have either a '2.' or '3.' prefix")
+		if latest != nil {
+			assert.True(t, strings.HasPrefix(latest.Version.String(), "2.") || strings.HasPrefix(latest.Version.String(), "3."),
+				"latest version of PMM should have either a '2.' or '3.' prefix")
+		}
 	})
 
 	t.Run("TestParseFile", func(t *testing.T) {