Input FORMAT of second fractions

Fractions has redundant trailing zeros in contrast to other part. Demonstrating query:

```
SQL> select cast('1-1-1 1:1:1.1' as timestamp format 'yyyy-mm-dd hh24:mi:ss.ff4'), timestamp'1-1-1 1:1:1.1' from rdb$database;

                     CAST                  CONSTANT
========================= =========================
0001-01-01 01:01:01.0001  2001-01-01 01:01:01.1000
```