I have found that valid XPath expressions apparently produce empty strings in EvaluateXPath processor which can be observed in the attached screenshots. I am using nifi 1.7.1 and jdk1.8.0_181.jdk. Any help would be appreciated!
The source of the XML files is a GetFile Processor or a GenrateFlowFile Processor.
I have also tested it with an InvokeHTTP processor where it works fine. However this does not solve it for me as I am dealing with physical XML files here. However this little detail might be interesting for the error finding process.