As delta uses parquet files to store data inside delta:

"Nullity is encoded in the definition levels (which is run-length encoded). NULL values are not encoded in the data. For example, in a non-nested schema, a column with 1000 NULLs would be encoded with run-length encoding (0, 1000 times) for the definition levels and nothing else."

source: https://parquet.apache.org/docs/file-format/nulls/