Deny pass `null` to `CreateService::getTemplate()`

[vjik]

null make no sense here.

⚠️ It breaks BC.

[samdark]

The reason was to have a nicer exception "You must define a template to generate the migration." instead of generic type mismatch.

public function getTemplate(?string $key): string
    {
        $key = (string) $key;

        if ($this->templates === null) {
            $this->setDefaultTemplates();
        }

        if (!isset($this->templates[$key])) {
            throw new InvalidConfigException('You must define a template to generate the migration.');
        }

        return $this->templates[$key];
    }

[vjik]

The reason was to have a nicer exception "You must define a template to generate the migration." instead of generic type mismatch.

null is never passed to this method.