Fix mysql sink error when inserted value is null

1. map null value to `null` but not `'null'` when generating sql.
2. optimize insert sql template from `INSERT INTO %s VALUES(%s)` to `INSERT INTO %s (%s) VALUES(%s)` to avoid enumerating all table columns.
3. [additional] remove tolowcase when checkFields method.

Start by locating the MySQL sink's SQL-generation and checkFields code; inspect how null values and columns are rendered. Verify that generated SQL uses SQL NULL rather than a quoted value, includes an explicit column list, and applies the requested case-handling change.

Written by the indexing model from the issue text.