Add null check in getShaderNodes

[jstone-lucasfilm]

This changelist adds a missing null check in getShaderNodes, handling the edge case of an invalid output string. Previously, this edge case would trigger a crash in MaterialXCore, and now it correctly generates validation warnings and proceeds.

[ld-kerley]

In cases like this where we find a bug I think we should add a unit test to exercise the fix at the same time as submitting the fix - that way we will try and ensure that we don't regress. Do you think we can add a small unit test here to simulate the bad input data?

[jstone-lucasfilm]

I was thinking along the same lines, @ld-kerley, though we'll need to create a bit more infrastructure to allow for "invalid" documents in our TestSuite. Two potential paths forward come to mind:

1. We could create a new "invalid" folder in our TestSuite, which would be loaded along with other examples during testing, but would not require a passing result in validation. As of today, we require all of our TestSuite examples to pass validation, so adding even a single invalid file would be a breaking change.
2. We could extend our fuzz testing system so that it consistently generates this category of error procedurally. This would require more work, but would potentially catch a much wider set of error conditions, not just the invalid output string that inspired this fix.