`ValueTracker.evaluate()` used `if not value` to skip NULL values, but
this also skipped empty strings (`""`), `0`, and other falsy values.
A CSV column containing only empty strings was incorrectly inferred as
INTEGER (the initial default) instead of TEXT.
Replace the falsy check with an explicit `if value is None` so that only
NULL is skipped and all other values — including `""` — are evaluated
against the type tests.
Queries returning duplicate column names - e.g. joins between tables
sharing column names - silently lost values because rows were built
with dict(zip(keys, row)), where the last duplicate wins.
Later occurrences are now renamed with a numeric suffix: id, id
becomes id, id_2 - skipping any suffix that would collide with a
real column in the same query.
The new utils.dedupe_keys() helper transforms the key list once per
query, so the per-row dict construction is unchanged and there is no
measurable performance impact.
Applied in Database.query() (including the PRAGMA and RETURNING
paths), Table.rows_where(), Table.search() and the CLI's JSON output.
CSV, TSV and table output keep the original duplicate headers.
Closes#624
