Don't set outputs if empty or nil

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

src/buildx.ts

@@ -14,7 +14,11 @@ export async function getMetadata(): Promise<string | undefined> {
   if (!fs.existsSync(metadataFile)) {
     return undefined;
   }
-  return fs.readFileSync(metadataFile, {encoding: 'utf-8'});
+  const content = fs.readFileSync(metadataFile, {encoding: 'utf-8'}).trim();
+  if (content === 'null') {
+    return undefined;
+  }
+  return content;
 }
 
 export async function isAvailable(): Promise<Boolean> {