enhancement(stdlib): Add support for null keys to object_from_array

[bruceg]

Summary

Last part of #1157, I think. Documentation will come in Vector once this is closed. This isn't strictly necessary but seemed to be a nice convenience to provide for optional fields.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance
• Enhancement

Is this a breaking change?

• Yes
• No

How did you test this PR?

Unit tests are included.

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.
• For new VRL functions, please also create a sibling PR in Vector to document the new function.

References

• feat(stdlib): Add new object_from_array function #1164
• enhancement(stdlib): Add support for keys parameter to object_from_array #1176