ADIF field should be filled with null in case of failure

assets/js/sections/common.js

@@ -177,7 +177,7 @@ function qso_edit(id) {
                                   document.getElementById("distance").value = data;
                                },
                                error: function() {
-                                  document.getElementById("distance").value = 'Error calculating distance!';
+                                  document.getElementById("distance").value = null;
                                },
                             });
                         }

assets/js/sections/qso.js

@@ -863,7 +863,7 @@ $("#locator").keyup(function(){
 					document.getElementById("distance").value = data;
 				},
 				error: function() {
-					document.getElementById("distance").value = 'Error calculating distance!';
+					document.getElementById("distance").value = null;
 				},
 			});
 		}