GH-51780: [C++][IPC] Remove the mislabeled null check on schema fields in GetSchema

[CaptainAni187]

Rationale for this change

In GetSchema, the loop over the schema's fields checked each Field for null with the label "DictionaryEncoding.indexType". That label belongs to a different check, so if this one ever failed, the error would point at the wrong field. As the XXX comment next to it said, the check also can't fail: for a vector of tables, fields()->Get(i) returns the element's address plus its stored offset, never null. Schema.fields itself is already checked for null just above.

What changes are included in this PR?

The check and its comment are removed, which is the first option in the issue. Child fields in FieldFromFlatbuffer are already read through children->Get(i) without a null check, so top-level fields now match.

Are these changes tested?

No new test, since the removed check couldn't fail. The arrow-ipc-* tests pass locally.

Are there any user-facing changes?

No.

Was AI used for this PR?

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PR code and description written by:

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Reviewed before submission by:

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• GitHub Issue: [C++] GetSchema labels its null check on each schema field as "DictionaryEncoding.indexType" #51780

[github-actions]

⚠️ GitHub issue #51780 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #51780 has been automatically assigned in GitHub to PR creator.

[pitrou]

I think this is ok, let's see whether CI is green.