Introspect zero defaults on SingleStore numeric columns

[greymoth-jp]

Introspecting (drizzle-kit pull) a SingleStore smallint, mediumint, bigint, float, double or real column whose DEFAULT is 0 drops the default from the generated schema.

Each of those branches in introspect-singlestore.ts guards .default() with a plain truthy check on the default value:

out += defaultValue
  ? `.default(${mapColumnDefault(defaultValue, isExpression)})`
  : '';

0 is falsy, so it is treated as "no default" and the generated TypeScript loses it, even though the introspected snapshot stores the 0 correctly. A fresh pull then stops matching the database. The int and tinyint branches in the same file already guard with typeof defaultValue !== 'undefined'; this applies the same check to the remaining numeric branches.

This is the SingleStore counterpart of #5917 (MySQL float/double/real) and #5918 (MySQL smallint/mediumint/bigint) — the SingleStore introspector has the same branches with the same truthy guard.

I scoped the change to the numeric types covered by those two PRs and left boolean untouched. I didn't add a test because the introspect suite starts a SingleStore container, which I couldn't run locally; happy to mirror the tests from #5917/#5918 if you'd prefer one here.