fix(common): correct and simplify typing of I18nPluralPipe (#37447)

I18nPluralPipe can actually accept `null` and `undefined` (which are
convenient for composing it with the async pipe), but it is currently
typed to only accept `number`.

PR Close #37447

packages/common/test/pipes/i18n_plural_pipe_spec.ts

@@ -54,12 +54,17 @@ import {JitReflector} from '@angular/platform-browser-dynamic/src/compiler_refle
       });
 
       it('should use "" if value is undefined', () => {
-        const val = pipe.transform(void (0) as any, mapping);
+        const val = pipe.transform(undefined, mapping);
+        expect(val).toEqual('');
+      });
+
+      it('should use "" if value is null', () => {
+        const val = pipe.transform(null, mapping);
         expect(val).toEqual('');
       });
 
       it('should not support bad arguments', () => {
-        expect(() => pipe.transform(0, <any>'hey')).toThrowError();
+        expect(() => pipe.transform(0, 'hey' as any)).toThrowError();
       });
     });
   });

packages/common/test/pipes/i18n_select_pipe_spec.ts

@@ -40,7 +40,7 @@ import {JitReflector} from '@angular/platform-browser-dynamic/src/compiler_refle
       });
 
       it('should throw on bad arguments', () => {
-        expect(() => pipe.transform('male', <any>'hey')).toThrowError();
+        expect(() => pipe.transform('male', 'hey' as any)).toThrowError();
       });
     });
   });