[5.8] Fix database rules with WHERE clauses

[staudenmeir]

exists and unique rules fail if a WHERE clause has an empty string value:

Validator::make(
    ['id' => 1],
    ['id' => Rule::exists('users')->where('name', '')]
)->passes();

This throws an exception:

Undefined offset: 1

The error is caused by rtrim(..., , ',') in Exists::__toString():

dump((string) Rule::exists('users')->where('name', ''));

// expected: exists:users,NULL,name,
// actual:   exists:users,NULL,name

The underlying issue is that ValidationRuleParser::parseParameters() parses parameters as CSV, but only receives plain text. This also affects values that can be interpreted as CSV:

Rule::exists('users')->where('name', '"foo"') // where "name" = 'foo'

We can fix it by converting all values to CSV as we do with in and notin rules (#21012).

Fixes #28697.

---

Theoretically, this can be a breaking change if someone works around the issue by providing CSV values:

 Rule::exists('users')->where('name', '""')

Do we consider cases like this?