fix(typeCast): TINYINT(1) null handling

[airbon99]

Problem reproduction

• Left joining a tinyint(1) field which doesn't have a match returns false

Expected behaviour

• Return null

[Kenshiin13]

Confirmed on my end, nice catch 👍🏽

The old field.string() === '1' evaluates null === '1' → false when the column is NULL (e.g. an unmatched LEFT JOIN), so a tinyint(1) that should be nil came back false. The change short-circuits NULL and only does the boolean compare when a value is present:

value	before	after
'1'	true	true
'0'	false	false
NULL	false	null
length ≠ 1	next()	next()

Two things to note:

BIT(1) has the same bug in the case right below:

case 'BIT':
  return field.length === 1 ? field.buffer()?.[0] === 1 : field.buffer()?.[0];

A NULL bit(1) gives field.buffer() → null → null?.[0] → undefined → undefined === 1 → false, identical to the TINY issue. Worth fixing in the same or a followup PR.

case 'BIT': {
  if (field.length !== 1) return field.buffer()?.[0] ?? null;
  const buf = field.buffer();
  return buf !== null ? buf[0] === 1 : null;
}

A regression test for "NULL tinyint(1) → null" would be good to have once #283 is merged.