Ensure parent term ID is non-negative in Term_Command (#662)

Co-authored-by: Claude <noreply@anthropic.com>

Author: swissspidy

src/Term_Command.php

@@ -590,7 +590,7 @@ public function generate( $args, $assoc_args ) {
 			}
 
 			$args = [
-				'parent' => $current_parent,
+				'parent' => max( 0, $current_parent ),
 				'slug'   => $slug . "-{$index}",
 			];