fix(app): App type def fix (#8365)

---------

Co-authored-by: Mike Hardy <github@mikehardy.net>

Author: MichaelVerdon

packages/app/lib/modular/index.d.ts

@@ -15,13 +15,13 @@ export function deleteApp(app: FirebaseApp): Promise<void>;
  * Registers a library's name and version for platform logging purposes.
  * @param libraryKeyOrName - Library name or key.
  * @param version - Library version.
- * @param variant - Library variant.
+ * @param variant - Library variant. Optional.
  * @returns Promise<void>
  */
 export function registerVersion(
   libraryKeyOrName: string,
   version: string,
-  variant: string | null,
+  variant?: string,
 ): Promise<void>;
 
 /**

packages/app/lib/modular/index.js

@@ -31,7 +31,7 @@ export function deleteApp(app) {
  * Registers a library's name and version for platform logging purposes.
   @param {string} libraryKeyOrName - library name or key.
   @param {string} version - library version.
-  @param {string | null} variant - library variant.
+  @param {string | undefined} variant - library variant. Optional.
  * @returns {Promise<void>}
  */
 export function registerVersion(libraryKeyOrName, version, variant) {