MDEV-12001 Split Item_func_round::fix_length_and_dec to virtual methods in Type_handler

This patch makes the following changes (according to the task description):
- Adds Type_handler::Item_func_round_fix_length_and_dec().
- Splits the code from Item_func_round::fix_length_and_dec() into new
  Item_func_round methods fix_arg_int(), fix_arg_decimal(), fix_arg_double().
- Calls the new Item_func_round methods from the relevant implementations of
  Type_handler_xxx::Item_func_round_fix_length_and_dec().
- Adds a new error message ER_ILLEGAL_PARAMETER_DATA_TYPE_FOR_OPERATION
- Makes ROUND() return the new error for GEOMETRY

Additionally:
- Inherits Item_func_round directly from Item_func_numhybrid as it
  uses nothing from Item_func_num1.
- Fixes "MDEV-12000 ROUND(expr,const_expr_returning_NULL) creates DOUBLE(0,0)".
  Now if args[1] returns NULL, the data type is set to DOUBLE with
  NOT_FIXED_DEC decimals instead of 0 decimals.

mysql-test/t/func_math.test

@@ -608,3 +608,19 @@ select 0=0, 0=-0, 0.0= -0.0, 0.0 = -(0.0), 0.0E1=-0.0E1, 0.0E1=-(0.0E1);
 --echo #
 
 select CRC32(NULL), CRC32(''), CRC32('MySQL'), CRC32('mysql'), CRC32('01234567'), CRC32('012345678');
+
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-12000 ROUND(expr,const_expr_returning_NULL) creates DOUBLE(0,0)
+--echo #
+
+CREATE OR REPLACE TABLE t1 AS SELECT
+  ROUND(10,NULL) AS c1,
+  ROUND(10.1,NULL) AS c2,
+  ROUND(10e0,NULL) AS c3;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;