Merge pull request #693 from vChavezB/label-fix

External Property Parser: Fix comparison to optional variable

src/main/java/widoco/ExternalPropertyParser.java

@@ -292,7 +292,7 @@ private Optional<OWLAnnotation> getLabelAnnotation(OWLEntity entity, String lang
                 continue;
             }
             label = getLabel(entity, langPrefix, importedOntology);
-            if (label != null) return label;
+            if (!label.isEmpty()) return label;
         }
         return Optional.empty();
     }