BUG: Fix unsetting of global properties

The set_property command unsets a property if it is given no value.  In
the case of GLOBAL properties, the cmake::SetProperty method would
replace a NULL value with "NOTFOUND".  Instead it should be left as NULL
so that the property is unset as expected.  Once it is unset the
get_cmake_property command will still report NOTFOUND while the
get_property command will return the empty string as documented.