Given:
```
<div class="s1" [class]="null" [ngClass]="exp">
```
Notice that `[class]` binding is not a `string`. As a result the existing logic would not concatenate `[class]` with `class="s1"`. The resulting falsy value would than be sent to `ngClass` which would promptly clear all styles on the `<div>`
The new logic correctly handles falsy values for `[class]` bindings.
Fix#35335
PR Close#35350