Changing that in case of *INT and hex hybrid input:
- ROUND(x,NULL) creates a column with the same type as x.
  The old code created a DOUBLE column, which was not relevant at all.
  This change simplifies the code a lot.

- ROUND(x,non_constant) creates a column of the INT, BIGINT or DECIMAL
  data type (depending on the exact type of x).
  The old code created a column of the DOUBLE data type,
  which lead to precision loss. Hence MDEV-23366.

- ROUND(bigint_30,negative_constant) creates a column of the DECIMAL(30,0)
  data type. The old code created DECIMAL(29,0), which looked strange:
  the data type promoted to a higher one, but max length reduced.
  Now the length attribute is preserved.