Fix nextKey skipping a key and throwing for keys not in the map

[rootvector2]

AbstractSortedMapDecorator.nextKey walks tailMap(key) and discards the first element to step over key itself, but never checks that key is present. When key is absent that first element is the successor, so it gets dropped and the wrong key comes back; when key is at or past the end tailMap is empty and it.next() throws NoSuchElementException instead of returning null. DualTreeBidiMap.nextKey repeats the same tailMap().iterator().next() shape and fails the same two ways, since its isEmpty() guard only covers a fully empty map.

Return null when the map does not contain key, matching the documented null if no match contract and every other nextKey in the library (AbstractLinkedMap, ListOrderedMap, TreeBidiMap, PatriciaTrie all return null for an absent key); the skip-first logic then runs only for a present key, the case it was written for. FixedSizeSortedMap and UnmodifiableSortedMap inherit the decorator fix. Found by auditing the OrderedMap.nextKey implementations against the contract.

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[garydgregory]

Hello @rootvector2

Thank you for the PR. How about adding tests for:

• An explicit test for DualTreeBidiMap.nextKey() with an absent key that is between existing keys. The abstract test uses getOtherKeys() which are outside the sample range, so the bug would still be hidden if the key were in-range.
• An explicit test for UnmodifiableSortedMap.nextKey(). It inherits the decorator fix but has no dedicated test.

Should the code be using NavigableMap.higherKey() when available? TreeMap implements NavigableMap, so higherKey(key) would give the correct successor in one call and would be null for absent keys automatically.

Should we guard the OrderedMap branch in DualTreeBidiMap.nextKey() for consistency; either ensure the delegated implementation also checks presence or apply the same containsKey guard before delegation.

Isn't there a similar problem with previousKey()? That could be addressed in a different PR.