Schema Nullability Validation Fails to Account for Dictionary Nulls

[tustvold]

Describe the bug

Currently various constructs containing nested Field validate that the nullability of their children matches that of the schema. This helps to avoid subtle bugs that could result in data loss / corruption, see #4602 for more context on this.

Currently this is achieved by inspecting the null count of the child array, however, for dictionaries this is potentially incorrect as the null count of the parent only reflects the null count of the keys, and not the logical null count.

To Reproduce

Expected behavior

Additional context

[tustvold]

label_issue.py automatically added labels {'parquet'} from #4691

[tustvold]

label_issue.py automatically added labels {'arrow'} from #4691