Update DepthFirstSearchIterative.js

[shrivalidutt]

Added a check at the start of DFSIterative to handle the case where the starting node doesn't exist. This prevents runtime errors and informs the user.

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Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not
• All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Project coverage is 84.64%. Comparing base (18da83a) to head (947b2b9).

Files with missing lines	Patch %	Lines
Graphs/DepthFirstSearchIterative.js	0.00%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1723      +/-   ##
==========================================
- Coverage   84.65%   84.64%   -0.02%     
==========================================
  Files         378      378              
  Lines       19744    19748       +4     
  Branches     2951     2951              
==========================================
  Hits        16715    16715              
- Misses       3029     3033       +4     

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[appgurueu]

See #1722. What this PR implements is also worse than throwing an error (side effect of logging), and it contains unnecessary style regressions.