The bug happened under the following condition:
  - there was a user variable of type REAL, containing NULL value
  - there was a table with a NOT_NULL column of any type but REAL, having
    default value (or auto increment);
  - a row was inserted into the table with the user variable as value.
    A warning was emitted here.

The problem was that handling of NULL values of REAL type was not properly
implemented: it didn't expect that REAL NULL value can be assigned to other
data type.

Basically, the problem was that set_field_to_null() was used instead of
set_field_to_null_with_conversions().

The fix is to use the right function, or more generally, to allow conversion of
REAL NULL values to other data types.