avoid call if there are no packages

Author: weaverryan

src/PackageJsonSynchronizer.php

@@ -253,6 +253,10 @@ private function updateImportMap(array $importMapEntries): void
             throw new \InvalidArgumentException(sprintf('Invalid importmap entry: "%s".', var_export($importMapEntry, true)));
         }
 
+        if (!$remotePackageNameAndConstraints) {
+            return;
+        }
+
         $this->scriptExecutor->execute(
             'symfony-cmd',
             'importmap:require',