keep an escaped value that equals the null string

[saleemno1]

CSVParser.handleNull compares the fully unescaped token against the format's null string, so a value that really is \N and that the printer correctly wrote as \\N for MYSQL, POSTGRESQL_TEXT, POSTGRESQL_CSV and ORACLE comes back as null; the lexer now records whether an escape sequence was actually translated, and only an untranslated token can be the null marker.

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