Test for warning about from_networkx 0-vector feature default (#897)
Test for warning about from_networkx 0-vector feature default (#897)

When reading features from an attribute in a node, the
`StellarGraph.from_networkx` functionality will implicitly fill in a 0 vector
for nodes that are missing the attribute. It does this with a warning, and so to
verify that this is behaving correctly and to avoid the warning appearing in our
test logs (for example:
https://buildkite.com/stellar/stellargraph-public/builds/1575#64220409-0450-476e-83c3-698926b32c10/254-413),
we can and should check for it using `pytest.warns`:
https://docs.pytest.org/en/latest/warnings.html#warns.
stellargraph/stellargraph,stellargraph/stellargraph