Orm: DDC-3825: simple_array slush with empty array

[doctrinebot]

Jira issue originally created by user bigfootdd:

I'm using "doctrine/orm": v2.5.0 and have simple_array type.
If I persist the class where the property with type simple_array is empty array() the query fails.

I'm coming with solution, please see the github PR https://github.com/doctrine/dbal/pull/877

[doctrinebot]

Comment created by bigfoot:

I've mapped the field as nullable=false, so it should never be null.
If you cannot merge this quickly, you should at least add an warning in the docs saying that the field must be mapped set as nullable.

[renanbr]

As alternative, I put filter in getter and setter methods:

public function setFields(array $values): void
{
    $this->fields = array_filter($values) ?: [null];
}

public function getFields(): array
{
   return array_filter($this->fields);
}

Note: It obviously doesn't work if you want to store empty values