Correctly fetch owners if metadata is falsey

[martosaur]

Currently, it is impossible to fetch owner if stored metadata is falsey:

iex(1)> {:ok, pid} = NimbleOwnership.start_link(name: Server)
{:ok, #PID<0.199.0>}
iex(2)> {:ok, nil} = NimbleOwnership.get_and_update(Server, self(), :foo, fn metadata -> {nil, metadata} end)
{:ok, nil}
iex(3)> NimbleOwnership.fetch_owner(Server, [self()], :foo)
:error

The PR fixes it by replacing chained map access in cond with matching case.

[josevalim]

💚 💙 💜 💛 ❤️

[whatyouhide]

Nice! Great cathc.