fix null pointer exception

filename were generated without '$', but the parsing code expected
the '$'.

pdb-ui/src/main/java/org/lucares/pdbui/TcpIngestor.java

@@ -142,7 +142,7 @@ public class TcpIngestor implements Ingestor, AutoCloseable, DisposableBean {
 				default:
 					if (value instanceof String) {
 						tags.add(key, (String) value);
-					} else {
+					} else if (value != null) {
 						tags.add(key, String.valueOf(value));
 					}
 					break;