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

goldens/public-api/common/common.d.ts

@@ -110,7 +110,7 @@ export declare class HashLocationStrategy extends LocationStrategy {
 
 export declare class I18nPluralPipe implements PipeTransform {
     constructor(_localization: NgLocalization);
-    transform(value: number, pluralMap: {
+    transform(value: number | null | undefined, pluralMap: {
         [count: string]: string;
     }, locale?: string): string;
 }

packages/common/src/pipes/i18n_plural_pipe.ts

@@ -39,7 +39,8 @@ export class I18nPluralPipe implements PipeTransform {
    * @param locale a `string` defining the locale to use (uses the current {@link LOCALE_ID} by
    * default).
    */
-  transform(value: number, pluralMap: {[count: string]: string}, locale?: string): string {
+  transform(value: number|null|undefined, pluralMap: {[count: string]: string}, locale?: string):
+      string {
     if (value == null) return '';
 
     if (typeof pluralMap !== 'object' || pluralMap === null) {

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();
       });
     });
   });