fix bug on s3 prefix stripping

Author: rongyamazon

src/stacks/actions/StackActionOperations.ts

@@ -54,7 +54,9 @@ export async function processChangeSet(
     let templateS3Url: string | undefined;
     try {
         if (params.s3Bucket) {
-            const s3KeyPrefix = params.s3Key ? params.s3Key.slice(0, params.s3Key.lastIndexOf('/')) : undefined;
+            const s3KeyPrefix = params.s3Key?.includes('/')
+                ? params.s3Key.slice(0, params.s3Key.lastIndexOf('/'))
+                : undefined;
             const template = new ArtifactExporter(s3Service, document.documentType, document.uri, document.contents());
 
             const exportedTemplate = await template.export(params.s3Bucket, s3KeyPrefix);