Answering my own question. Solved with an if else statement using has_excerpt.
if( has_excerpt ( $_post, ID ) ) {
$author = sprintf( '<a href="https://wordpress.stackexchange.com/questions/224893/%1$s">Apply</a>', esc_attr( get_the_excerpt( $_post ) ) );
} else {
$author="";
}
It only makes sense, really.