When the UR data_type is missing, assume varchar

This works well enough for sqlite and postgres, but not oracle.
A later commit should add the capability for proper conversion of
UR data type to data-source-specific data type (the converse of
ur_data_type_for_data_source_data_type(), including an appropriate
fallback data type that will accept anything)

Change-Id: Ie5f5e33567380e6311b6821bc02e08e24a08a9dc