Treat empty query param as absent.

With this fix, both
	```dynazoom.html?cgiurl_graph=&plugin_name=...```
and
	```dynazoom.html?plugin_name=```
now fall back to the default value for ```cgiurl_graph```.

This is needed to make fallback to the default path work if the user
hasn't specified it explicitly in ```/etc/munin.conf```. Previously, the
empty string would have been used as path for ```cgiurl_graph```.