[Variant] Avoid collecting offset iterator

[friendlymatthew]

Related to #7878

map_bytes_to_offsets is a utility that transforms a byte buffer into an iterator of offsets, based on a given size.

It is used in several places throughout the validation code.

In all cases, we can avoid materializing the iterator with .collect() by instead looping over it directly and moving the relevant checks into that loop

[codephage2020]

take

[alamb]

label_issue.py automatically added labels {'parquet'} from #7934