Optimize traversing empty map

[Caulm]

Eliminate performance overhead in constructing begin() iterators on empty hash tables.

When iterating over an empty phmap::flat_hash_map, begin() unnecessarily called skip_empty_or_deleted() on the first bucket, causing measurable overhead.

This fix aligns with the approach already taken in Abseil: if the map is empty, simply return end(). See raw_hash_set.h#L2355

Benchmark (traversing empty map, capacity 100M, seconds):

std::unordered_map	Original phmap::flat_hash_map	Fixed phmap::flat_hash_map
0.000	0.047	0.000

[greg7mdp]

Nice, thanks for the PR @Caulm !