Connected to Oracle Database 12c Enterprise Edition Release 12.1.///
Connected as ///

SQL> 
SQL> Create Table Def_Null_Test (n Number);

Table created


SQL> Create Table Def_Null_Log (Seq Number, Def_Val Clob);

Table created


SQL> Insert Into Def_Null_Log
  2    (Seq, Def_Val)
  3    Select 0, To_Lob(Data_Default)
  4      From User_Tab_Columns
  5     Where Table_Name = 'DEF_NULL_TEST';

1 row inserted


SQL> Alter Table Def_Null_Test Modify n Default Null;

Table altered


SQL> Insert Into Def_Null_Log
  2    (Seq, Def_Val)
  3    Select 1, To_Lob(Data_Default)
  4      From User_Tab_Columns
  5     Where Table_Name = 'DEF_NULL_TEST';

1 row inserted


SQL> Alter Table Def_Null_Test Modify (n Default Null);

Table altered


SQL> Insert Into Def_Null_Log
  2    (Seq, Def_Val)
  3    Select 2, To_Lob(Data_Default)
  4      From User_Tab_Columns
  5     Where Table_Name = 'DEF_NULL_TEST';

1 row inserted


SQL> Select Seq, Dump(Dbms_Lob.Substr(Def_Val, 4000)) Def_Val_Len
  2    From Def_Null_Log
  3   Order By Seq;

       SEQ DEF_VAL_LEN
---------- --------------------------------------------------------------------------------
         0 NULL
         1 Typ=1 Len=5: 78,117,108,108,10
         2 Typ=1 Len=4: 78,117,108,108

SQL> Drop Table Def_Null_Test;

Table dropped


SQL> Drop Table Def_Null_Log;

Table dropped


SQL>