Fix: regression when opening a new File

[AdarshdeepCheema]

This regression was caused by https://github.com/zowe/zlux-editor/pull/285/files#diff-168aee0d0ff778647f681a145fb1ce371d7f68f2ca9a30f014aa3f3efc214a56
For new files the path did not exist and the exception was thrown. Now it is fixed

Proposed changes

This PR addresses Issue: [Link to Github issue within https://github.com/zowe/zlux/issues if any]

This PR depends upon the following PRs:

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Change in a documentation
• Refactor the code
• Chore, repository cleanup, updates the dependencies.
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

PR Checklist

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• If the changes in this PR are meant for the next release / mainline, this PR targets the "staging" branch.
• My code follows the style guidelines of this project (see: Contributing guideline)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes
• video or image is included if visual changes are made
• Relevant update to CHANGELOG.md
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works, or describe a test method below

Testing

Further comments

[DivergentEuropeans]

MVS and USS file tab handling appears to work well, in combo with hotkeys, closing, saving, and diff checking so approving and merging thanks Adarsh