How can I implode an array while skipping empty array items?

You can use array_filter():

If no callback is supplied, all entries of input equal to FALSE (see converting to boolean) will be removed.

implode('-', array_filter($array));

Obviously this will not work if you have 0 (or any other value that evaluates to false) in your array and you want to keep it. But then you can provide your own callback function.