fix: edge_index.dtype is float

[aMahanna]

The __process_adb_edge_df method is responsible for creating & appending to an EdgeStorage's edge_index.

Part of this code is responsible for mapping the ArangoDB Edges _from and _to values to the ArangoDB Vertex Collection Map (adb_map) to preserve the zero-based index value associated to the _from and _to IDs.

For a reason (not yet know), this mapping is returning a Pandas Series of floats instead of integers. These values are then piped into the edge_index property, resulting in edge_index.dtype being float...

This PR aims to address this issue by invoking astype(int) after invoking the Pandas map() function.

• However, invoking astype(int) on a Pandas Series with possible NaN values will raise, so we must also introduce fillna(-1). This in turn affects the handling of Invalid Edges, hence why that code was also modified

[aMahanna]

notice how just adding the assertions from 2980240 will fail the CI

[geenen124]

lgtm