Fix reading required string values inside an optional group

[adamreeve]

Fixes #480.

When string values are required and inside an optional nested group, the group is skipped over in LogicalBatchReaderFactory and we only use a single LeafReader instance to read values. This should read null into destination rows where the enclosing group is null. But when _nullableLeafValues is false in the BufferedReader, only the non-null values would be read so we'd reach the end of the values before reaching the expected number of rows/levels.

We don't have the same problem for a required int inside an optional group for example as this is detected in GetCompoundReader when typeof(TElement) != typeof(TLogical) and is handled specially with the OptionalReader class.

[marcin-krystianc]

LGTM 👍