[PM-24105] Remove usage of getUserKey on keyService (#16626)

• prefer undefined over null
• obtain required UserId once per method, before branching
• guards moved to beginning of methods
* lift UserId retrieval to occur once during import
* remove redundant userId retrieval

apps/browser/src/background/main.background.ts

@@ -990,6 +990,7 @@ export default class MainBackground {
 
     this.sendStateProvider = new SendStateProvider(this.stateProvider);
     this.sendService = new SendService(
+      this.accountService,
       this.keyService,
       this.i18nService,
       this.keyGenerationService,