* in node graph, fix edges dataframe miscategorization
A dataframe named "edges" could end up getting categorized as a nodes dataframe if it was missing a "source" field, resulting in a very confusing error message "id field is required for nodes dataframe" instead of a more sensible error message about the missing source field.
* Fix lint
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Co-authored-by: Andrej Ocenas <mr.ocenas@gmail.com>
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