Enable WP_Query to search by "standard" post format.

query_format_standard.php

<?php

function akv3_query_format_standard($query) {
	if (isset($query->query_vars['post_format']) &&
		$query->query_vars['post_format'] == 'post-format-standard') {
		if (($post_formats = get_theme_support('post-formats')) &&
			is_array($post_formats[0]) && count($post_formats[0])) {
			$terms = array();
			foreach ($post_formats[0] as $format) {
				$terms[] = 'post-format-'.$format;
			}
			$query->is_tax = null;
			unset($query->query_vars['post_format']);
			unset($query->query_vars['taxonomy']);
			unset($query->query_vars['term']);
			unset($query->query['post_format']);
			$query->set('tax_query', array(
				'relation' => 'AND',
				array(
					'taxonomy' => 'post_format',
					'terms' => $terms,
					'field' => 'slug',
					'operator' => 'NOT IN'
				)
			));
		}
	}
}
add_action('pre_get_posts', 'akv3_query_format_standard');