Fix TS diagnostics not being cleared propery

Fixes #50401

extensions/typescript-language-features/src/features/diagnostics.ts

@@ -105,8 +105,6 @@ export class DiagnosticsManager {
 			return;
 		}
 
-		collection.set(file, diagnostics);
-
 		if (diagnostics.length === 0) {
 			const existing = collection.get(file);
 			if (existing.length === 0) {
@@ -115,6 +113,8 @@ export class DiagnosticsManager {
 			}
 		}
 
+		collection.set(file, diagnostics);
+
 		this.scheduleDiagnosticsUpdate(file);
 	}