Default UUID values affecting data insertions by not generating a random UUID when null parameters are passed

Fix for https://github.com/geonetwork/core-geonetwork/issues/291.

I noticed when I attempt to insert a WMS GetCapabilities XML, it calls the XSL file without a uuid parameter set.  This results in the metadata ID being set to the string literal "uuid".  Because this string's length > 0, it never gets a new randomly generated UUID in the Java code (in the Insert class).  An easy fix is to just remove the placeholder value so that it is set to the empty string in the XSL and then assigned a random value after the transformation.