Python: Fix ModuleVariableNode.toString

In some cases mod.getName() does not have a result, so toString of
ModuleVariableNode would also not have a result, which would cause
data-flow paths that use these as an edge to not be valid :O

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -407,7 +407,7 @@ class ModuleVariableNode extends Node, TModuleVariableNode {
   override Scope getScope() { result = mod }
 
   override string toString() {
-    result = "ModuleVariableNode for " + mod.getName() + "." + var.getId()
+    result = "ModuleVariableNode in " + mod.toString() + " for " + var.getId()
   }
 
   /** Gets the module in which this variable appears. */