Add doctests to DFS and BFS. Contributes to #9943

[prak74]

Describe your change:

I have added doctests to some functions inside directed_and_undirected_graphs.py file as part of the Hacktoberfest 2024 contributions. I have also improved the dfs and bfs code to better handle edge cases, such as passing -2 as a node, running dfs, bfs on empty graphs, etc. Contributes to #9943

• [ ] Add an algorithm?
• [x] Fix a bug or typo in an existing algorithm?
• [x] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• [ ] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [ ] All function parameters and return values are annotated with Python type hints.
• [ ] All functions have doctests that pass the automated testing.
• [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[prak74]

@cclauss @tianyizheng02 can you kindly take a look?