checker: fix option mismatch checking on array initializations(fix #20409)

[shove70]

1. Fixed C error when using an ?string member of a struct as a parameter to pg exec function #20409
2. Add tests.

When the array is initialized, V takes the type of the first element as the type of the array elements, so when the first type is not option, the following elements should not have option either.

import db.pg

@[table: 'demo']
struct Demo {
	id int @[primary; sql: serial]
	a string
	b ?string
}

fn run() ! {
	conn := '' // insert own connection string
	db := pg.connect_with_conninfo(conn)!
	sql db { create table Demo }!
	demo := Demo{0, 'a', none}
	_ := db.exec_param_many('INSERT INTO demo(a, b) VALUES($1, $2)', [demo.a, demo.b])!
	db.close()
}

run() or { panic(err.msg()) }

outputs:

a.v:15:81: error: cannot use `?string` as `string`, it must be unwrapped first
   13 |     sql db { create table Demo }!
   14 |     demo := Demo{0, 'a', none}
   15 |     _ := db.exec_param_many('INSERT INTO demo(a, b) VALUES($1, $2)', [demo.a, demo.b])!
      |                                                                                    ^
   16 |     db.close()
   17 | }

[spytheman]

Excellent work.