Fix test after upgrade

This is a workaround to, if not a bug, at least a behavior change in Grails
2.3.0.

Doing this:
	mockDomain(Plugin);
	def p = new Plugin(
			...
			defaultLink: '',
			hasModules: false)

results in p.defaultLink == null instead of ''. Worked around by doing
p.defaultLink = '' after the constructor call.