fix(material/chips): emit null instead of undefined when deselecting the only selected chip

src/material/chips/chip-listbox.spec.ts

@@ -652,6 +652,26 @@ describe('MatChipListbox', () => {
             .toEqual('steak-0');
         });
 
+        it('should propagate null, not undefined, when the selected chip is deselected', () => {
+          dispatchKeyboardEvent(primaryActions[0], 'keydown', SPACE);
+          fixture.detectChanges();
+
+          expect(fixture.componentInstance.control.value)
+            .withContext(`Expected control's value to be set to the new option.`)
+            .toEqual('steak-0');
+
+          dispatchKeyboardEvent(primaryActions[0], 'keydown', SPACE);
+          fixture.detectChanges();
+
+          expect(fixture.componentInstance.control.value)
+            .withContext(
+              `Expected control's value to be null after deselecting the only selected ` +
+                `chip, not undefined - consumers (e.g. Angular Signal Forms) treat a value of ` +
+                `undefined as "this field no longer exists" rather than "this field is empty".`,
+            )
+            .toEqual(null);
+        });
+
         it('should clear the selection when a nonexistent option value is selected', () => {
           const array = chips.toArray();
 

src/material/chips/chip-listbox.ts

@@ -321,7 +321,7 @@ export class MatChipListbox
     if (Array.isArray(this.selected)) {
       valueToEmit = this.selected.map(chip => chip.value);
     } else {
-      valueToEmit = this.selected ? this.selected.value : undefined;
+      valueToEmit = this.selected ? this.selected.value : null;
     }
     this._value = valueToEmit;
     this.change.emit(new MatChipListboxChange(this, valueToEmit));