Issue with avg. untyped String literal length measure

[LorenzBuehmann]

https://github.com/SANSA-Stack/SANSA-RDF/blob/2669c80a1dbdcfc419897d1b51877b4fb0ddd38d/sansa-rdf-spark/src/main/scala/net/sansa_stack/rdf/spark/stats/RDFStatistics.scala#L256-L261

this code will not work because since RDF 1.1 - which is default mode in Apache Jena - every literal has a datatype. Previous untyped literals (aka plain literals) now do have the datatype rdf:langString. That's also the reason why in the unit test it always returns 0.0 although the data contains e.g.

<http://example.org/show/218> <http://example.org/show/localName> "That Seventies Show"@en . # literal with a language tag