fix(langgraph): treat clear_cache([]) as no-op instead of clearing all

[zhgao]

Closes #8960

Pregel.clear_cache([]) was treated the same as clear_cache(None), wiping every node's cache, because both the sync and async variants used nodes = nodes or self.nodes.keys(), which conflates an explicit empty list with "not given". Per the BaseCache contract, only None means "all nodes"; an explicit empty list should clear nothing.

Two-line fix: only fall back to all nodes when nodes is None. clear_cache(None) behavior is unchanged.

Tests: reproduced with the issue's own repro script (before: clear_cache([]) triggered recomputation; after: no-op for [], still clears all for None); existing cache tests green locally.

AI assistance disclosure: prepared with AI assistance; reproduced and verified locally.

[github-actions]

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