Fix nested elements sorting

Author: mariuszfoltak

src/DataTable.spec.ts

@@ -291,8 +291,9 @@ describe("DataTable directive tests", ()=> {
         it("should sort data by child property value", ()=> {
             let newData = [
                 {name: 'Claire', city: {zip: '51111'}},
-                {name: 'Anna', city: {zip: '31111'}},
+                {name: 'Anna'},
                 {name: 'Claire', city: {zip: '41111'}},
+                {name: 'Donald', city: 2},
                 {name: 'Claire', city: {zip: '11111'}},
                 {name: 'Anna', city: {zip: '21111'}}
             ];
@@ -303,9 +304,10 @@ describe("DataTable directive tests", ()=> {
             expect(datatable.data).toEqual([
                 {name: 'Claire', city: {zip: '11111'}},
                 {name: 'Anna', city: {zip: '21111'}},
-                {name: 'Anna', city: {zip: '31111'}},
                 {name: 'Claire', city: {zip: '41111'}},
                 {name: 'Claire', city: {zip: '51111'}},
+                {name: 'Anna'},
+                {name: 'Donald', city: 2},
             ]);
         });
     });

src/DataTable.ts

@@ -153,7 +153,9 @@ export class DataTable implements OnChanges, DoCheck {
         return (row: any): any => {
             var value = row;
             for (let sortByProperty of sortBy.split('.')) {
-                value = value[sortByProperty];
+                if(value) {
+                    value = value[sortByProperty];
+                }
             }
             if (value && typeof value === 'string' || value instanceof String) {
                 return value.toLowerCase();