D: Fix TResourcePool.Range.willBecomeNonempty for empty pools.

This is mostly just for good feelings, as a client pool without
any members isn't something a Thrift user would ever need to
work correctly.
D: Minor code cleanup in thrift.internal.resource_pool.

Because of oddities in the standard library at the time Thrift/D
was written the code was a bit uglier than necessary.