constellation

Analysis: The decimals is set to NOT_FIXED_DEC for Field_str even if it is
NULL. Unsigned has decimals=0. So Type_std_attributes::decimals is set to 39
(maximum between 0 and 39). This results in incorrect number of decimals
when we have union of unsigned and NULL type.

Fix: Check if the field is created from NULL value. If yes, set decimals to 0
otherwise set it to NOT_FIXED_DEC.