Fix null pointer dereference in tf2xla ParseShardingFromEdgeSource
Wasn't it fixed by https://github.com/tensorflow/tensorflow/pull/58912? @apach301, yeah, my bad, this PR is a duplicate.
The bug was found by Svace static analyzer:

1. edge.src() is null
2. edge.DebugString() dereferences src_ (that is null) via src_->name()

cc @mihaimaruseac