fix(common): allow null/undefined to be passed to ngClass input (angu…

…lar#39280)

With strict template type checking, a null/undefined value will raise an
error. However the implementation is completely fine with it, and it
would be pointless to "fix" it at the callsite and convert to e.g. an
empty string. Allow all of the "supported types" to be passed in
directly to ngClass.

Fixes angular#39280

goldens/public-api/common/index.md

@@ -403,7 +403,7 @@ export class NgClass implements DoCheck {
     // (undocumented)
     set ngClass(value: string | string[] | Set<string> | {
         [klass: string]: any;
-    });
+    } | null | undefined);
     // (undocumented)
     ngDoCheck(): void;
     // (undocumented)

packages/common/src/directives/ng_class.ts

@@ -61,7 +61,7 @@ export class NgClass implements DoCheck {
   }
 
   @Input('ngClass')
-  set ngClass(value: string|string[]|Set<string>|{[klass: string]: any}) {
+  set ngClass(value: string|string[]|Set<string>|{[klass: string]: any}|null|undefined) {
     this._removeClasses(this._rawClass);
     this._applyClasses(this._initialClasses);