[Validator] Allow checkMX() to return false when $host is empty
[Validator] Allow checkMX() to return false when $host is empty

| Q | A |
| --- | --- |
| Branch? | master |
| Bug fix? | yes |
| New feature? | no |
| BC breaks? | no |
| Deprecations? | no |
| Tests pass? | yes |
| Fixed tickets | #22106 |
| License | MIT |
| Doc PR | n/a |
symfony/Validator