If your nodes only have a next and no prev reference, then null-ifying first and last references will clear the list, IF your values don't hold a value to the nodes.
This means:
Type Queue has reference to first:node, last:node
Node has reference next:node, value:object
the value objects don't have any reference to either of both.
This means:
Type Queue has reference to first:node, last:node
Node has reference next:node, value:object
the value objects don't have any reference to either of both.