fix graph bug when first position has no data

When there is no json data, the function will stop before recoding the
altitude in the first series. Subquential position would cause an
exception.

Author: rossengeorgiev

js/tracker.js

@@ -1543,11 +1543,10 @@ function graphAddPosition(vcallsign, new_data) {
 
     // we don't record extra data, if there is no telemetry graph loaded
     // altitude is used for altitude profile
-    if(plot) {
+    if(plot && new_data.data != "") {
 
         // the rest of the series is from the data field
         var json = $.parseJSON(new_data.data);
-        if(!json) return;
 
         // init empty data matrix
         var data_matrix = [];