[BUG] Make U+0000 (NUL) work with Postgres

[georgeh0]

U+0000 (NUL) is valid in Unicode, but unfortunately it's not supported by Postgres (Postgres documentation).
Sometimes the input text has U+0000 (usually test cases to intentionally contain control characters, example), and the flow will fail when writing into Postgres.

Our plan:

• For Postgres target, sanitize and strip U+0000, document the behavior clearly.
• For data written into our internal storage (tracking table), specifically the cached intermediate data, escape U+0000 before writing it down, and unescape it at read time.

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