Update DepthFirstTraversal.java

[sanvi-05]

Input validation has been added for both the dfs method to check if the vertex is within the valid range and the initEdges method to ensure that the number of vertices is greater than 0. If the input is invalid, it throws an IllegalArgumentException with an appropriate error message.

Fixes #{ISSUE_NUMBER}

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[raj-j-shah]

@sanvi-05 Please refrain from sending pull requests directly. You must first be assigned to a task, after which your pull request will be reviewed and, if approved, merged.