Fix NLA action cannot be unlinked in certain cases

The poll for unlinking calls `nla_panel_context` without providing an
adt pointer, and there is a check for this pointer in
`nla_panel_context` leading to never returning true if it is not
provided. (this is fine if there are tracks already, poll would succeed
in this case, `nla_panel_context` goes a different code path then)

Same call to `nla_panel_context` is also done in the beginning of the
corresponding unlink exec function (but this time providing the pointer
because it is used later), so it makes sense to do the same thing in the
poll function. Equal check is also done in the panel poll function, so
now these are all in sync.

Part of T87681.

Maniphest Tasks: T87681

Differential Revision: https://developer.blender.org/D11041

source/blender/editors/space_nla/nla_channels.c

@@ -562,7 +562,8 @@ void NLA_OT_action_pushdown(wmOperatorType *ot)
 static bool nla_action_unlink_poll(bContext *C)
 {
   if (ED_operator_nla_active(C)) {
-    return nla_panel_context(C, NULL, NULL, NULL);
+    PointerRNA adt_ptr;
+    return (nla_panel_context(C, &adt_ptr, NULL, NULL) && (adt_ptr.data != NULL));
   }
 
   /* something failed... */