Support submitting array of empty values as GeoPoint attribute value

[Pictor13]

AggregateRootTypeCommandValidator could be improved in its default behaviour when dealing with a GeoPoint attribute.

At the moment we need to explicitly specify, in the entity schema:

<attribute name="coords" type="geo-point">
    <option name="null_island_as_null">false</option>
    <option name="default_value">
        <option name="lon">0.0</option>
        <option name="lat">0.0</option>
    </option>
</attribute>

Otherwise when submitting empty strings for lon and lat values the validation will always fail (because an array of empty values is submitted as value for the attribute, while Trellis just checks for NULL, empty-string, or null-island, as accepted NULL values).

The AggregateRootTypeCommandValidator should consider this case and set then a proper value, taking in account the option null_island_as_null.
I am not sure though how to deal with that in the case where just one of the two coordinates is specified.

[MrHash]

This should be handled by modifying the Trellis GeopointAttributeRule to cast empty string coordinate arrays to the null value.