Fix nx -> pyg conversion with no edges

[anton-bushuiev]

Reference Issues/PRs

Hi, @a-r-j ! I have spotted a minor corner case bug. Currently nx -> pyg conversion fails on index out range error when input graph has no edges.

What does this implement/fix? Explain your changes

What testing did you do to verify the changes in this PR?

Pull Request Checklist

• [x] Added a note about the modification or contribution to the ./CHANGELOG.md file (if applicable)
• [ ] Added appropriate unit test functions in the ./graphein/tests/* directories (if applicable)
• [x] Modify documentation in the corresponding Jupyter Notebook under ./notebooks/ (if applicable)
• [x] Ran python -m py.test tests/ and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., python -m py.test tests/protein/test_graphs.py)
• [x] Checked for style issues by running black . and isort .

[sonarcloud[bot]]

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[a-r-j]

Thanks for the contribution!