Fix Can't remove a slot from dynamips device

Fix https://github.com/GNS3/gns3-gui/issues/1510

gns3server/compute/dynamips/__init__.py

@@ -520,7 +520,7 @@ class Dynamips(BaseManager):
                         yield from vm.slot_add_binding(slot_id, adapter)
                 except IndexError:
                     raise DynamipsError("Slot {} doesn't exist on this router".format(slot_id))
-            elif name.startswith("slot") and value is None:
+            elif name.startswith("slot") and (value is None or value is ""):
                 slot_id = int(name[-1])
                 try:
                     if vm.slots[slot_id]:
@@ -538,7 +538,7 @@ class Dynamips(BaseManager):
                         yield from vm.install_wic(wic_slot_id, wic)
                 except IndexError:
                     raise DynamipsError("WIC slot {} doesn't exist on this router".format(wic_slot_id))
-            elif name.startswith("wic") and value is None:
+            elif name.startswith("wic") and (value is None or value is ""):
                 wic_slot_id = int(name[-1])
                 try:
                     if vm.slots[0].wics and vm.slots[0].wics[wic_slot_id]: