implement get_block_args method in Acf_Json_Block

When extending `Acf_Json_Block` class, block registration data are
provided by the `block.json` file. The class implement the method
to return an empty array.

classes/Blocks/Acf_Json_Block.php

@@ -26,4 +26,13 @@ public function register(): void {
 			]
 		);
 	}
+
+	/**
+	 * Block registration args loaded from `block.json` file.
+	 *
+	 * @return array
+	 */
+	public function get_block_args(): array {
+		return [];
+	}
 }