Delta validity is a top level child

For performant delta encoding we have to read all the values in deltas
buffer that means that undefined values in deltas buffer are
significant. Instead of performing careful transformation dance to
ensure correctness we remove null values from deltas and keep them as
a top level child. Cascading encodings don't need any special handling
and will preserve all values since they're all valid.

Signed-off-by: Robert Kruszewski <github@robertk.io>

Author: robert3005