Fix undefined bug

Author: benibenj

src/vs/workbench/services/editor/common/customEditorLabelService.ts

@@ -40,7 +40,7 @@ export class CustomEditorLabelService extends Disposable implements ICustomEdito
 	private patterns: ICustomEditorLabelPattern[] = [];
 	private enabled = true;
 
-	private cache = new MRUCache<string, string | undefined>(1000);
+	private cache = new MRUCache<string, string | null>(1000);
 
 	constructor(
 		@IConfigurationService private readonly configurationService: IConfigurationService,
@@ -123,11 +123,11 @@ export class CustomEditorLabelService extends Disposable implements ICustomEdito
 		const key = resource.toString();
 		const cached = this.cache.get(key);
 		if (cached !== undefined) {
-			return cached;
+			return cached ?? undefined;
 		}
 
 		const result = this.applyPatterns(resource);
-		this.cache.set(key, result);
+		this.cache.set(key, result ?? null);
 
 		return result;
 	}