/**
 * Converts an image string to a data URL if it is not already in a recognized format.
 *
 * This function processes an input string representing an image and converts it to a data URL if necessary. The function handles several cases:
 * - If the input is already a data URL or an HTTP/HTTPS URL, it is returned unchanged.
 * - If the input is a base64 encoded string, it is converted to a data URL with the MIME type 'image/png'.
 * - If the input is an empty string or not a string, it is returned unchanged.
 *
 * Args:
 *     image_str (str): The input string representing the image. This can be a data URL, HTTP/HTTPS URL, or a base64 encoded string.
 *
 * Returns:
 *     str: The input string converted to a data URL if necessary, or the original string if it is already in a recognized format.
 *
 * Examples:
 *     >>> to_data_url('data:image/png;base64,iVBORw0KGgo')
 *     'data:image/png;base64,iVBORw0KGgo'
 *     >>> to_data_url('http://example.com/image.png')
 *     'http://example.com/image.png'
 *     >>> to_data_url('iVBORw0KGgoabc123')
 *     'data:image/png;base64,iVBORw0KGgoabc123'
 *     >>> to_data_url('randomstring')
 *     'data:image/png;base64,randomstring'
 */