Fix warning in Connection::tryDeserializeError()

[drunken-monkey]

Description

Fixes code in \OpenSearch\Connections\Connection::tryDeserializeError() to properly check whether $error['error']['root_cause'] exists before accessing it.

Issues Resolved

Closes #167.

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[dblock]

@drunken-monkey Your second commit is missing DCO, please?

[codecov[bot]]

Codecov Report

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

Comparison is base (9ebb183) 24.19% compared to head (cc67d75) 24.19%.

:exclamation: Current head cc67d75 differs from pull request most recent head dbd796f. Consider uploading reports for the commit dbd796f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #168 +/- ## ============================================ - Coverage 24.19% 24.19% -0.01% + Complexity 1938 1936 -2 ============================================ Files 246 246 Lines 6865 6862 -3 ============================================ - Hits 1661 1660 -1 + Misses 5204 5202 -2 ```

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[drunken-monkey]

@drunken-monkey Your second commit is missing DCO, please?

Sorry, fixed!

[dblock]

Do add a line to CHANGELOG. This fixes a bug.

[dblock]

@drunken-monkey You have to amend your last commit to pass DCO, please?

[j-fulbright]

Can this get brought in? It is causing headaches with AWS OSS

[dblock]

Need another maintainer to approve. @VachaShah @shyim @harshavamsi ?