Return 1 byte length for zero len values when determining spaceForLength

[ou05020]

bits.Len returns 0 when len == 0. That results in a zero result returned by spaceForLength due to integer division (6/7). The writeLength function is then called using 1 byte to write the zero length value leading to a discrepancy between reserved (0) and written (1) number of bytes.

This causes incorrect values to be returned for empty strings in Stringbank.Get.