refactor(workers): Strict nulls for CommitMessage class (#12863)

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>

lib/workers/repository/model/commit-message.ts

 export class CommitMessage {
   public static readonly SEPARATOR: string = ':';
 
-  private message: string;
+  private message = '';
 
-  private prefix: string;
+  private prefix = '';
 
   constructor(message = '') {
     this.setMessage(message);
@@ -26,11 +26,11 @@ export class CommitMessage {
   }
 
   public setCustomPrefix(prefix?: string): void {
-    this.prefix = (prefix || '').trim();
+    this.prefix = (prefix ?? '').trim();
   }
 
   public setSemanticPrefix(type?: string, scope?: string): void {
-    this.prefix = (type || '').trim();
+    this.prefix = (type ?? '').trim();
 
     if (scope?.trim()) {
       this.prefix += `(${scope.trim()})`;

tsconfig.strict.json

@@ -122,6 +122,7 @@
     "./lib/workers/pr/changelog/hbs-template.ts",
     "./lib/workers/pr/changelog/types.ts",
     "./lib/workers/repository/init/types.ts",
+    "./lib/workers/repository/model/commit-message.ts",
     "./test/graphql-snapshot.ts",
     "./test/json-schema.ts",
     "./test/newline-snapshot-serializer.ts",