AttributeError crash in async exclude_edges for heterographs

[VbhvGupta]

When using the asynchronous version of exclude_edges (or exclude_seed_edges) on a heterogeneous graph, the program crashes if the set of edges to exclude doesn't cover all edge types present in the SampledSubgraph.

The _ExcludeEdgesWaiter.wait() method unconditionally calls .wait() on its internal futures, but for edge types with no edges to exclude, the value is None, which leads to an _AttributeError: 'NoneType' object has no attribute 'wait_'.

dgl/python/dgl/graphbolt/sampled_subgraph.py

Lines 36 to 37 in 3d16000

	for k in list(index.keys()):
	index[k] = index[k].wait()

It iterates through its internal dictionary and calls .wait() on every value, but it does not check if a value is None first.

A simple fix would be to add a if value is not None: check before the call.

[github-actions]

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