fixed broken tests

src/main/java/it/unibz/inf/isochrone/db/Database.java

@@ -783,6 +783,10 @@ public class Database {
 	 * @return a string with the placeholders plugged in to a statement.
 	 */
 	private static String calculatePlaceHolders(final int length) {
+		if (length <= 0) {
+			return "";
+		}
+
 		final StringBuilder builder = new StringBuilder((2 * length) - 1);
 		builder.append("?");