refactor(core): allow AnimationClassBindingFn to return undefined or null

The AnimationClassBindingFn type was too restrictive, only allowing `string | string[]`. However, the runtime (`getClassListFromValue`) safely handles `undefined` and `null` values by treating them as no animation.

This change updates the type to allow `undefined` and `null`, which is consistent with other class/style bindings in Angular and avoids requiring workarounds (like empty strings) in host bindings.

Added a compliance test case to verify that `[animate.enter]` with a potentially `undefined` value compiles correctly.

(cherry picked from commit a7bde662c3)

packages/compiler-cli/test/compliance/test_cases/r3_compiler_compliance/components_and_directives/GOLDEN_PARTIAL.js

@@ -787,3 +787,34 @@ export declare abstract class AbstractComp {
     static ɵcmp: i0.ɵɵComponentDeclaration<AbstractComp, "test-comp", never, {}, {}, never, never, true, never>;
 }
 
+/****************************************************************************************************
+ * PARTIAL FILE: host_animate_enter.js
+ ****************************************************************************************************/
+import { Component } from '@angular/core';
+import * as i0 from "@angular/core";
+export class TestCmp {
+    disabled = false;
+    static ɵfac = i0.ɵɵngDeclareFactory({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: TestCmp, deps: [], target: i0.ɵɵFactoryTarget.Component });
+    static ɵcmp = i0.ɵɵngDeclareComponent({ minVersion: "14.0.0", version: "0.0.0-PLACEHOLDER", type: TestCmp, isStandalone: true, selector: "test-cmp", host: { properties: { "animate.enter": "disabled ? undefined : 'enter-class'" } }, ngImport: i0, template: '', isInline: true });
+}
+i0.ɵɵngDeclareClassMetadata({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: TestCmp, decorators: [{
+            type: Component,
+            args: [{
+                    selector: 'test-cmp',
+                    template: '',
+                    host: {
+                        '[animate.enter]': "disabled ? undefined : 'enter-class'",
+                    }
+                }]
+        }] });
+
+/****************************************************************************************************
+ * PARTIAL FILE: host_animate_enter.d.ts
+ ****************************************************************************************************/
+import * as i0 from "@angular/core";
+export declare class TestCmp {
+    disabled: boolean;
+    static ɵfac: i0.ɵɵFactoryDeclaration<TestCmp, never>;
+    static ɵcmp: i0.ɵɵComponentDeclaration<TestCmp, "test-cmp", never, {}, {}, never, never, true, never>;
+}
+

packages/compiler-cli/test/compliance/test_cases/r3_compiler_compliance/components_and_directives/TEST_CASES.json

@@ -179,6 +179,22 @@
         }
       ],
       "compilationModeFilter": ["full compile", "instruction compile"]
+    },
+    {
+      "description": "should support host binding with animate.enter",
+      "inputFiles": ["host_animate_enter.ts"],
+      "expectations": [
+        {
+          "failureMessage": "Incorrect generated output",
+          "files": [
+            {
+              "expected": "host_animate_enter.js",
+              "generated": "host_animate_enter.js"
+            }
+          ]
+        }
+      ],
+      "compilationModeFilter": ["full compile", "instruction compile"]
     }
   ]
 }

packages/compiler-cli/test/compliance/test_cases/r3_compiler_compliance/components_and_directives/host_animate_enter.js

@@ -0,0 +1,24 @@
+import { Component } from '@angular/core';
+import * as i0 from "@angular/core";
+
+…
+export class TestCmp {
+  disabled = false;
+  …
+  static ɵcmp = /*@__PURE__*/ i0.ɵɵdefineComponent({
+    type: TestCmp,
+    selectors: [["test-cmp"]],
+    hostBindings: function TestCmp_HostBindings(rf, ctx) {
+      if (rf & 1) {
+        i0.ɵɵanimateEnter(function TestCmp_HostBindings_animateenter_cb() {
+          return ctx.disabled ? undefined : "enter-class";
+        });
+      }
+    },
+    decls: 0,
+    vars: 0,
+    template: function TestCmp_Template(rf, ctx) { },
+    encapsulation: 2
+  });
+}
+…

packages/compiler-cli/test/compliance/test_cases/r3_compiler_compliance/components_and_directives/host_animate_enter.ts

@@ -0,0 +1,12 @@
+import {Component} from '@angular/core';
+
+@Component({
+  selector: 'test-cmp',
+  template: '',
+  host: {
+    '[animate.enter]': "disabled ? undefined : 'enter-class'",
+  }
+})
+export class TestCmp {
+  disabled = false;
+}

packages/core/src/animation/interfaces.ts

@@ -94,4 +94,4 @@ export interface AnimationLViewData {
 /**
  * Function that returns the class or class list binded to the animate instruction
  */
-export type AnimationClassBindingFn = () => string | string[];
+export type AnimationClassBindingFn = () => string | string[] | undefined | null;