Python: Don't flag return procedure_call() in __init__ as error

This commit fixes the results for
0d8a429b7e/files/mayaTools/cgm/lib/classes/AttrFactory.py (L90)

```
def __init__(...):
    if error_case:
        return guiFactory.warning(...)
```

that was wrongly reporting _Explicit return in __init__ method._ as an error.

python/ql/src/Functions/ExplicitReturnInInit.ql

@@ -12,11 +12,12 @@
 
 import python
 
-from Return r
+from Return r, Expr rv
 where
-    exists(Function init | init.isInitMethod() and r.getScope() = init and exists(r.getValue())) and
-    not r.getValue() instanceof None and
-    not exists(FunctionValue f | f.getACall() = r.getValue().getAFlowNode() | f.neverReturns()) and
+    exists(Function init | init.isInitMethod() and r.getScope() = init) and
+    r.getValue() = rv and
+    not rv.pointsTo(Value::none_()) and
+    not exists(FunctionValue f | f.getACall() = rv.getAFlowNode() | f.neverReturns()) and
     // to avoid double reporting, don't trigger if returning result from other __init__ function
-    not exists(Attribute meth | meth = r.getValue().(Call).getFunc() | meth.getName() = "__init__")
+    not exists(Attribute meth | meth = rv.(Call).getFunc() | meth.getName() = "__init__")
 select r, "Explicit return in __init__ method."