[Issue #1374] distinguish SQL NULL from empty values across LOAD, column vectors, and Retina

[gengdy1545]

Summary

Core: only a null reference is SQL NULL; empty bytes/strings are empty values. This rule is now consistent end-to-end.

• Null detection: checkBytesNull treats only null as NULL (byte[0] is a value); BinaryColumnVector no longer coerces empty to NULL.
• LOAD: \N → NULL; empty text/binary → ""; empty other types → NULL.
• Retina wire: InsertData/UpdateData gain isNull to distinguish NULL from empty bytes; server maps to Java null, MemTable uses addNull().
• Primary key: NULL is rejected via PrimaryKeyBytes, shared by LOAD and Storage GC.

Also (supporting fixes): split BOOLEAN/TINYINT parsing; split(limit=-1) keeps trailing empties; LOAD rolls back and cleans unpublished files on failure.

[gengdy1545]

PTAL