'nullvalue STRING: Print STRING in place of null values
This affects textual output modes like column and list and
sets how SQL null values are shown.  The default is a zero
length string.  Insert mode and dumps are not affected by this
setting.  You can use double quotes to supply a zero length
string.  For example:
.nullvalue ""         # the default
.nullvalue <NULL>     # rather obvious
.nullvalue " \t "     # A tab surrounded by spaces'

def command_nullvalue(self, cmd):

    if (len(cmd) != 1):
        raise self.Error('nullvalue    takes    exactly    one    parameter')
    self.nullvalue = self.fixup_backslashes(cmd[0])
