Add special handling for empty strings for Bloom filter predicate push down

Updated hash computation for empty strings in the FastHash implementation to conform with the handling in Apache Impala. For Bloom filter predicate pushdown feature that uses FastHash, this makes the Kudu client older than version 1.15.0 incompatible with Kudu server version 1.15.0 and Kudu client version at or newer than 1.15.0 incompatible with Kudu server version earlier than 1.15.0. Both client library and Kudu server need to be updated to version 1.15.0 or above if using the Bloom filter predicate feature. Manifestations of this incompatibility are following messages in the logs: - "Not implemented: call requires unsupported application feature flags: 4". - "Not implemented: call requires unsupported application feature flags: 5".