fix(exporter.js): exporterExcelCustomFormatter should return the unmodified docDefinition

* Default exporterExcelCustomFormatter shouldn't return null

The default exporterExcelCustomFormatter custom function currently returns null which causes errors downstream. It should return the unmodified docDefinition

This should fix issue #6774

* Correcting the fix

The fix is supposed to be for exporterExcelCustomFormatter not exporterExcelHeader

closes #6774

Author: sujeetpillai

src/features/exporter/js/exporter.js

@@ -608,7 +608,7 @@
            *   }
            * </pre>
            */
-          gridOptions.exporterExcelCustomFormatters = gridOptions.exporterExcelCustomFormatters ? gridOptions.exporterExcelCustomFormatters : function( grid, workbook, docDefinition ) { return null; };
+          gridOptions.exporterExcelCustomFormatters = gridOptions.exporterExcelCustomFormatters ? gridOptions.exporterExcelCustomFormatters : function( grid, workbook, docDefinition ) { return docDefinition; };
 
           /**
            * @ngdoc function