fix null pointer exception

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

data-store/src/main/java/org/lucares/pdb/datastore/internal/FolderStorage.java

@@ -45,11 +45,11 @@ public class FolderStorage {
 
 		ensureCapacity();
 
-		String filename = filenamePrefix + filenameSuffix;
+		String filename = filenamePrefix + "$" + filenameSuffix;
 		int index = 1;
 		Path newFile = currentDirectory.resolve(filename);
 		while (Files.exists(newFile)) {
-			filename = filenamePrefix + index++ + filenameSuffix;
+			filename = filenamePrefix + "$" + index++ + filenameSuffix;
 			newFile = currentDirectory.resolve(filename);
 		}
 		Files.createFile(newFile);