Resolve inconsistency between objectclass and datatype

[clausnagel]

The objectclass metadata table offers an "Undefined" entry (id = 1). It can be used for storing features in the feature table whose feature type is not explicitly listed in objectclass. So, in such cases, the objectclass_id column can be set to 1.

The datatype metadata table does not offer a corresponding value. If an unknown datatype is used in the property table, the datatype_id column must therefore be set to null.

We should be consistent here. Either add an "Undefined" entry to the datatype table or remove it from objectclass. Since users are allowed to store null values in both objectclass_id and datatype_id anyway, we could easily define that null means "undefined" and, thus, remove the "Undefined" entry from objectclass to solve the issue.

[clausnagel]

Btw, the same issue exists for the namespace metadata table. It neither has an "Undefined" entry so that the namespace_id column must be set to null for unknown namespaces.

[clausnagel]

Added an Undefined entry in datatype with 89fad2d. Using null remains possible. Due to the two Undefined entries, changes to namespace are not required.