Defaults needed for Sequences

[cjmyers]

The encoding and elements values are optional in SBOL3 but required in SBOL2, so we need defaults when converting from SBOL3 to SBOL2.

[goksel]

Using some defaults may not work. In SBOL3, the encoding is required if the elements are provided. However, you can have the encoding property with no elements. How can we convert this SBOL2? How about not creating SBOL2.Sequences if both elements and encoding properties are not included in SBOL3.sequences?

[cjmyers]

Seems reasonable. Another approach is to assign empty string to elements and create ontology term for no encoding provided.