Don't display "group by size" if value is actually null or empty

+ add spec

app/assets/javascripts/admin/line_items/controllers/line_items_controller.js.coffee

@@ -219,7 +219,7 @@ angular.module("admin.lineItems").controller 'LineItemsCtrl', ($scope, $timeout,
 
   $scope.getGroupBySizeFormattedValueWithUnitName = (value, unitsProduct, unitsVariant) ->
     scale = $scope.getScale(unitsProduct, unitsVariant)
-    if scale
+    if scale && value
       value = value / scale if scale != 28.35 && scale != 1 && scale != 453.6 # divide by scale if not smallest unit
       $scope.getFormattedValueWithUnitName(value, unitsProduct, unitsVariant, scale)
     else

spec/javascripts/unit/admin/line_items/controllers/line_items_controller_spec.js.coffee

@@ -348,6 +348,19 @@ describe "LineItemsCtrl", ->
           spyOn(VariantUnitManager, "getUnitName").and.returnValue "lb"
           expect(scope.formattedValueWithUnitName(2, unitsProduct, unitsVariant)).toEqual "2 lb"
 
+      describe "get group by size formatted value with unit name", ->
+        beforeEach ->
+          spyOn(VariantUnitManager, "getUnitName").and.returnValue "kg"
+        
+        unitsProduct = { variant_unit: "weight", variant_unit_scale: 1000 }
+         
+        it "returns the formatted value with unit name", ->
+          expect(scope.getGroupBySizeFormattedValueWithUnitName(1000, unitsProduct)).toEqual "1 kg"
+
+        it "handle the case when the value is actually null or empty", ->
+          expect(scope.getGroupBySizeFormattedValueWithUnitName(null, unitsProduct)).toEqual ""
+          expect(scope.getGroupBySizeFormattedValueWithUnitName("", unitsProduct)).toEqual ""
+
 
       describe "updating the price upon updating the weight of a line item", ->
         beforeEach ->