Remove warning on attribute-less milestone

Custom milestones do not need to have attributes, therefore remove the
warning and only warn in case there is really other content than
attributes inside a milestone.

biblemulticonverter/src/main/java/biblemulticonverter/format/paratext/USFM.java

@@ -389,7 +389,7 @@ private ParatextBook doImportBook(File inputFile, Charset charset) throws Except
 				if (textPart.startsWith("|")) {
 					String attList = textPart.substring(1).trim();
 					parseAttributeList(attList, milestone.getAttributes());
-				} else {
+				} else if (!textPart.isEmpty()){
 					System.out.println("WARNING: Skipping unsupported milestone content: "+textPart);
 				}
 				pos = data.indexOf('\\', startPos + 2);