Fix nx -> pyg conversion with no edges (#334)

* Fix nx -> pyg conversion corner case when graph has no edges

* Update changelog

CHANGELOG.md

@@ -12,6 +12,9 @@
 * [Bugfix] - [#305](https://github.com/a-r-j/graphein/pull/305) Fixes the construction of geometric features when beta-carbons or side chains are missing in non-glycine residues (for example in `H:CYS:104` in 3SE8).
 * [Bugfix] - [#305](https://github.com/a-r-j/graphein/pull/305) Fixes data types of geometric feature vectors: `object` -> `float`.
 * [Bugfix] - [#301](https://github.com/a-r-j/graphein/pull/301) Fixes the conversion of undirected NetworkX graph to directed PyG data.
+* [Bugfix] - [#334](https://github.com/a-r-j/graphein/pull/334) Fixes the corner case of the NetworkX -> PyG conversion when input graph has no edges.
+
+https://github.com/a-r-j/graphein/pull/334
 
 #### Bugfixes
 * Adds missing `stage` parameter to `graphein.ml.datasets.foldcomp_data.FoldCompDataModule.setup()`. [#310](https://github.com/a-r-j/graphein/pull/310)

graphein/ml/conversion.py

@@ -289,7 +289,8 @@ def convert_nx_to_pyg(self, G: nx.Graph) -> Data:
                     data[key].append(value)
 
         # Add edge features
-        edge_feature_names = list(G.edges(data=True))[0][2].keys()
+        edge_list = list(G.edges(data=True))
+        edge_feature_names = edge_list[0][2].keys() if edge_list else []
         edge_feature_names = list(
             filter(
                 lambda x: x in self.columns and x != "kind", edge_feature_names