[5.8] Fix unique validation without ignored column

[staudenmeir]

#27940 broke unique validation without an ignored column:

$v = Validator::make(['email' => 'test@example.com'], ['email' => Rule::unique('users')]);
$v->passes();

// expected
select count(*) as aggregate from "users" where "email" = 'test@example.com'

// actual
select count(*) as aggregate from "users" where "email" = 'test@example.com' and "id" <> ''

We can only apply stripslashes() to non-null values.

The tests didn't catch this because Mockery only uses loose comparison.

Fixes #27986.