fix: LRUCache.get() and __getitem__() fail for cached None values

[koteshyelamati]

Problem

LRUCache.get() and LRUCache.__getitem__() both break when None is a legitimate cached value.

get() bug: uses if value is not None: to detect cache hits, so if a key maps to None, it skips the LRU promotion and returns the default instead of the cached None.

__getitem__() bug: calls self.get(key) and then checks if value is None: raise KeyError, so it raises KeyError even when the key exists with value None.

cache = LRUCache(capacity=5)
cache['key'] = None

cache.get('key', 'missing')  # Returns 'missing' instead of None
cache['key']                  # Raises KeyError instead of returning None

Root cause

Both methods use the cached value itself as a sentinel to detect cache misses. This is the same class of bug that was fixed in set() by PR #597.

Fix

Replace the value-based checks with key not in self.cache membership tests, consistent with how set() was already fixed.