Create an excerpt from the content if nothing is set

helper.php

/**
 * Allows to switch between excerpt and content.
 * If excerpt is not set we will use content but limit the words.
 * Ideally use for custom queries.
 *
 * @param string $content
 * @param string $excerpt
 * @param int $word_limit
 * @param string $more
 * @return string $trimmed_content
 */
function content_switch($content, $excerpt, $word_limit = '', $more = ' ... ')
{
  if ( empty($excerpt) ) 
  {
	$excerpt = wpautop( $content );
	$excerpt = substr( $excerpt, 0, strpos( $excerpt, '</p>' ) + 4 );
	$excerpt = strip_tags($excerpt, '<a><strong><em>');

	if (is_int($word_limit) && $word_limit > 0)
		$excerpt = wp_trim_words($excerpt, $word_limit, $more);
  }
		
  return apply_filters(  __NAMESPACE__ . '\\trim_content', $excerpt);
}