Bug: Empty langString causes ["Invalid URL: value"] error

[Yucukof]

(This is consequent to this change in shex-codegen)

Whenever a TTL file contains a rdfs:langString with an empty character sequence, such as the following

@prefix : <http://www.w3.org/ns/shapetrees#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .

:describesInstance
  a owl:ObjectProperty ;
  rdfs:domain :Description ;
  rdfs:comment ""@en ; // <- empty string
  rdfs:isDefinedBy <> ;
  rdfs:label "Describes Instance"@en .

then, the following error is thrown

    TypeError: Invalid URL: value
        at new URLImpl (/home/hba/Documents/Git/SOLID/src/node_modules/whatwg-url/dist/URL-impl.js:21:13)
        at Object.exports.setup (/home/hba/Documents/Git/SOLID/src/node_modules/whatwg-url/dist/URL.js:54:12)
        at new URL (/home/hba/Documents/Git/SOLID/src/node_modules/whatwg-url/dist/URL.js:107:22)
        at normalizeUrl (/home/hba/Documents/Git/shex-methods/lib/transform/rdfToData.js:86:23)
        at getNormalizedKeyFromContextOrSchemaPrefixes (/home/hba/Documents/Git/shex-methods/lib/transform/rdfToData.js:74:38)
        at /home/hba/Documents/Git/shex-methods/lib/transform/rdfToData.js:38:28
        at Array.map (<anonymous>)
        at absoluteToNormalized (/home/hba/Documents/Git/shex-methods/lib/transform/rdfToData.js:34:51)
        at /home/hba/Documents/Git/shex-methods/lib/transform/rdfToData.js:48:35
        at Array.map (<anonymous>)

      at ../../shex-methods/lib/validate.js:65:17

I have not been able to track the exact source of the issue, though.