[Variant] `metadata` field should be marked is non-nullable

[alamb]

metadata is non-nullable so the tests should set the nullable argument to false

Originally posted by @scovich in #8379 (comment)

[alamb]

label_issue.py automatically added labels {'parquet'} from #8379

[alamb]

label_issue.py automatically added labels {'parquet-variant'} from #8379