fix(Tinebase/js/grid): handle empty json in renderer

Change-Id: If0180986f2add380d6116662a0c554558e2a4ce6
Reviewed-on: http://gerrit.tine20.com/customers/18531
Tested-by: Jenkins CI (http://ci.tine20.com/) <tine20-jenkins@metaways.de>
Reviewed-by: Cornelius Weiss <c.weiss@metaways.de>

tine20/Tinebase/js/widgets/grid/jsonRenderer.js

@@ -22,8 +22,8 @@ Tine.widgets.grid.jsonRenderer.displayField = function(value) {
     return Tine.widgets.grid.jsonRenderer.syntaxHighlight(result);
 };
 
-Tine.widgets.grid.jsonRenderer.syntaxHighlight =function(json) {
-    json = json.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
+Tine.widgets.grid.jsonRenderer.syntaxHighlight = function(json) {
+    json = String(json).replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
     return json.replace(/("(\\u[a-zA-Z0-9]{4}|\\[^u]|[^\\"])*"(\s*:)?|\b(true|false|null)\b|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?)/g, function (match) {
         var cls = 'number';
         if (/^"/.test(match)) {
@@ -39,4 +39,4 @@ Tine.widgets.grid.jsonRenderer.syntaxHighlight =function(json) {
         }
         return '<span class="' + cls + '">' + match + '</span>';
     });
-};
+};