Merge pull request #23 from PathVisio/pubref-issue

fix for publication reference issue

Author: mkutmon

org.pathvisio.lib/src/main/java/org/pathvisio/libgpml/model/GPML2013aReader.java

@@ -365,6 +365,12 @@ protected void readPublicationXrefs(PathwayElement pathwayElement, Element e,
 				// reads PublicationXref/citation
 				String biopaxId = readPublicationXrefInfo(pubxf.getChildren("ID", BIOPAX_NAMESPACE));
 				String biopaxDb = readPublicationXrefInfo(pubxf.getChildren("DB", BIOPAX_NAMESPACE));
+				if(biopaxId == null || biopaxId.equals("")) {
+					biopaxId = "NA";
+				}
+				if(biopaxDb == null || biopaxDb.equals("")) {
+					biopaxDb = "PubMed";
+				}
 				Xref xref = XrefUtils.createXref(biopaxId, biopaxDb);
 				String source = readPublicationXrefInfo(pubxf.getChildren("SOURCE", BIOPAX_NAMESPACE));
 				// if source is an url, also set as citation urlLink