If no author posts, echo out some text

Maybe this do the trick, put your if statement inside the function to check if the number of posts retrieved is >= 1, if so begin the foreach else print a message

        function get_related_author_posts() {
            global $authordata, $post;

            $authors_posts = get_posts( array( 'author' => $authordata->ID, 'post_type' => 'kenniscentrum', 'post__not_in' => array( $post->ID ), 'posts_per_page' => 5 ) );

            if( $authors_posts && count( $authors_posts ) >= 1 ) :
            $output="<ul>";
            foreach ( $authors_posts as $authors_post ) {
                $output .= '<li><a href="' . get_permalink( $authors_post->ID ) . '">' . apply_filters( 'the_title', $authors_post->post_title, $authors_post->ID ) . '</a>' . apply_filters( 'the_excerpt', $authors_post->post_excerpt, $authors_post->ID ) . '</li>';
            }
            $output .= '</ul>';
            else :
            $output="<p>". __( 'No posts found' ) .'</p>';
            endif;
            return $output;

        }