contains and keys were giving inconsistent results

[ljeub-pometry]

What changes were proposed in this pull request?

Fixes a panic exception in python for constant properties

    g = Graph()
    g.add_edge(0, 1, 2, layer="a")
    g.add_edge(0, 1, 2)
    g.edge(1, 2).add_constant_properties({"test": 1})
    constant_exploded = g.layer("a").edges.explode().properties.constant.values() # used to panic here!

Why are the changes needed?

bug due to keys and contains having inconsistent results

Does this PR introduce any user-facing change? If yes is this documented?

No

How was this patch tested?

Added a test for the above example in python

Are there any further changes required?

[miratepuffin]

LGTM