Handle empty KUBECONFIG path item gracefully when other valid paths

[marcodlk]

fixes #369

Changes:

• Update KubeConfig.__init__:
  • raise ValueError if empty string or None provided
  • raise IsDirectoryError if path provided is a directory. Error message similar to kubectl

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.29%. Comparing base (87063fc) to head (c23d2d4). Report is 105 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #378 +/- ## ========================================== + Coverage 94.61% 95.29% +0.67% ========================================== Files 29 29 Lines 3141 3446 +305 ========================================== + Hits 2972 3284 +312 + Misses 169 162 -7 ```

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