fix: omit absent compileSerializationSchema metadata instead of passing null - #7001

Closes #6988.

compileSerializationSchema defaulted both optional parameters to null:

function (schema, httpStatus = null, contentType = null)

so a custom serializer compiler called through reply.compileSerializationSchema(schema) received null for both. FastifyRouteSchemaDef declares them as optional strings and the reference docs describe the default as undefined, so the implementation was the odd one out. serializeInput already declares the same two parameters without defaults and forwards undefined, so this also makes the two paths agree.

Dropping the defaults is the whole change. Existing tests that touch this assert !httpStatus rather than a specific value, so they hold either way, and I could not find anywhere in lib, test or types that compares this to null.

Added one test asserting both values arrive as undefined when the metadata is not supplied. It fails without the change.

Verified locally: node --test test/internals/*.test.js gives 395 passed / 0 failed, npm run test:types passes all 16 files and 1282 assertions, and eslint is clean on both touched files.

I left a separate note on #6987 about the cache-keying half, since fixing that one has a public API consequence for getSerializationFunction and I would rather agree the approach first.