I found this recently with a 6-field form. The browser sends empty FORM fieldstrings.

Eg: http://www.bing.com/search?q=test&qs=n&form=QBLH&...

Here, sk is "".

You can strip out empty fields with a bit of PHP (creating a new URL without the empty strings), which seems to work okay, but it's probably best not to risk the wrong effect, which is the end program seeing the empty fields wrongly (as NULL and not "").