Fix memory leak in TypeHandlerBase.DictionaryKeyComparer.Equals

[ratacolita]

The bug was in line 22 of TypeHandlerBase.cs: if (x != null && y != null) return false;

This condition returns false when BOTH objects are non-null, which is the normal comparison case. This caused the ConcurrentDictionary to:

1. Never find existing cache entries (Equals always returns false)
2. Add a new entry for every SetValue() call
3. Grow unboundedly causing memory leaks in production

The fix changes the null checks to proper equality semantics: if (x == null && y == null) return true; // Both null = equal if (x == null || y == null) return false; // One null = not equal

Added unit test to verify cache reuse behavior.

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