Title: fix: FrozenConfigDict crashes with empty tuple before list (#61)

[dcaayushd]

Fixes #61
empty tuple before a list breaks FrozenConfigDict, reverse order works fine.

root cause: empty tuples are singletons in Python so id(()) is always
the same. when a second empty sequence hits _tuple_to_immutable, the
id is already in visit_map and it hits the assert.

fix: return early for empty tuples before touching visit_map since
theres nothing to iterate over anyway.

added a regression test for both orderings.

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[dcaayushd]

@mohitreddy1996 please take a look at this.