fix(ivy): narrow NgIf context variables in template type checker (#35125)

When the `NgIf` directive is used in a template, its context variables
can be used to capture the bound value. This is typically used together
with a pipe or function call, where the resulting value is captured in a
context variable. There's two syntax forms available:

1. Binding to `NgIfContext.ngIf` using the `as` syntax:
```html
<span *ngIf="(user$ | async) as user">{{user.name}}</span>
```

2. Binding to `NgIfContext.$implicit` using the `let` syntax:
```html
<span *ngIf="user$ | async; let user">{{user.name}}</span>
```

Because of the semantics of `ngIf`, it is known that the captured
context variable is non-nullable, however the template type checker
would not consider them as such and still report errors when
`strictNullTypes` is enabled.

This commit updates `NgIf`'s context guard to make the types of the
context variables non-nullable, avoiding the issue.

Fixes #34572

PR Close #35125

packages/common/src/directives/ng_if.ts

@@ -232,7 +232,9 @@ export class NgIf<T = unknown> {
    * The presence of this method is a signal to the Ivy template type-check compiler that the
    * `NgIf` structural directive renders its template with a specific context type.
    */
-  static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<T> { return true; }
+  static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<NonNullable<T>> {
+    return true;
+  }
 }
 
 /**