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.

Source/cmake.cxx

@@ -3550,10 +3550,6 @@ void cmake::SetProperty(const char* prop, const char* value)
     {
     return;
     }
-  if (!value)
-    {
-    value = "NOTFOUND";
-    }
 
   this->Properties.SetProperty(prop, value, cmProperty::GLOBAL);
 }