Fix ability run input handling without schema

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>

Author: Copilot

features/ability.feature

@@ -191,6 +191,19 @@ Feature: Manage abilities registered via the WordPress Abilities API.
               },
               'permission_callback' => '__return_true',
           ) );
+
+          wp_register_ability( 'test-plugin/simple-no-input', array(
+              'label'               => 'Simple No Input',
+              'description'         => 'Returns an empty array.',
+              'category'            => 'test-category',
+              'output_schema'       => array(
+                  'type' => 'array',
+              ),
+              'execute_callback'    => function() {
+                  return array();
+              },
+              'permission_callback' => '__return_true',
+          ) );
       } );
       """
 
@@ -224,6 +237,12 @@ Feature: Manage abilities registered via the WordPress Abilities API.
       {"sum":30}
       """
 
+    When I run `wp ability run test-plugin/simple-no-input`
+    Then STDOUT should be:
+      """
+      []
+      """
+
     When I run `wp ability run test-plugin/get-site-title --format=yaml`
     Then STDOUT should contain:
       """

src/Ability_Command.php

@@ -593,7 +593,7 @@ private function build_input( $assoc_args ) {
 	 * Builds input data from associative arguments with stdin support.
 	 *
 	 * @param array $assoc_args Associative arguments.
-	 * @return array<string,mixed> The input data.
+	 * @return array<string,mixed>|null The input data, or null if none provided.
 	 */
 	private function build_input_with_stdin( $assoc_args ) {
 		$input = [];
@@ -626,7 +626,8 @@ private function build_input_with_stdin( $assoc_args ) {
 			$input[ $key ] = $value;
 		}
 
-		return $input;
+		// Return null if no input provided (for abilities without input schema).
+		return empty( $input ) ? null : $input;
 	}
 
 	/**