Check if a field is empty
If you read up on the documentation the function returns FALSE if there is no data. isset() would actually fail because you are assigning FALSE to the variable so it's actually been set to something. The correct syntax would be:

$field = field_get_items('node', $node, 'field_post_image');
if ($field) {
   //Do something with the field
}
else{
   //There are no results
}