Check for null fill value when deserializing (#1782)
Check for null fill value when deserializing (#1782)

This prevents the following error when deserializing from the current
REST server:
[error] [TileDB::Attribute] Error: Cannot set fill value; Input value cannot be null

Co-authored-by: Joe Maley <16a9a54ddf4259952e3c118c763138e83693d7fd@tiledb.com>
TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB,TileDB-Inc/TileDB