"Not a valid (absolute) URI" on Literal columns without a datatype

[vcutrona]

Annotating a column as a Literal (either from the Tabular Annotation and RDF mappings tabs) without specifying the datatype causes the runtime error "Not a valid (absolute) URI: cell value".

How to reproduce:

1. Create a new transformation and upload a file
2. Switch to the RDF mapping tab and select the "Custom mapping" tab in the left panel
3. Add a triple URI - prop - Literal, without enabling the "Assign data type flag"
4. Save the pipeline
   4.1 Downloading the RDF triples will return an empty file
   4.2 Downloading and executing the JAR transformation will throw the error above

Note: The W3C recommends to always specify a datatype for literals (RDF 1.1 spec), but also specifies that "... concrete syntaxes MAY support simple literals consisting of only a lexical form without any datatype IRI or language tag. Simple literals are syntactic sugar for abstract syntax literals with the datatype IRI http://www.w3.org/2001/XMLSchema#string."
We can thus support the simple literals or resort to setting the xsd:string datatype by default.