fix(bug): fixed bug with ensuring the metadata_mapping value

[IvanOvchynnikov]

Description

I fixed the bug with validation of metadata_mapping on None type. Moved the reference to the first element after validation. Changes are present on line 1056.
Version of llama-index: 0.11.16

Fixes # (issue)
Validation after referencing.

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods