fix(query-builder): prevent error when entities are empty and expressionTree is set

getFormat() was calling .pipeArgs.format without optional chaining after
find(), causing Angular's ngFormatDate to receive undefined as the format
string (TypeError: Cannot read properties of undefined (reading 'length'))
when fields is null due to an empty entities array. Added full optional
chaining and a DEFAULT_PIPE_DATE_FORMAT fallback.

Author: simeonoff

projects/igniteui-angular/query-builder/src/query-builder/query-builder-tree.component.ts

@@ -1380,7 +1380,7 @@ export class IgxQueryBuilderTreeComponent implements AfterViewInit, OnDestroy {
      * @hidden @internal
      */
     public getFormat(field: string) {
-        return this.fields?.find(el => el.field === field).pipeArgs.format;
+        return this.fields?.find(el => el.field === field)?.pipeArgs?.format ?? DEFAULT_PIPE_DATE_FORMAT;
     }
 
     /**

projects/igniteui-angular/query-builder/src/query-builder/query-builder.component.spec.ts

@@ -28,6 +28,7 @@ describe('IgxQueryBuilder', () => {
         IgxQueryBuilderComponent,
         IgxQueryBuilderSampleTestComponent,
         IgxQueryBuilderCustomTemplateSampleTestComponent,
+        IgxQueryBuilderInvalidSampleTestComponent,
         IgxComboComponent
       ]
     }).compileComponents();