backport: Convert DATETIME to TIME type error

[hustjieke]

Is your feature request related to a problem? Please describe.

working issue: #995
commit id: 2a03448

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Additional context

[Double0101]

When datetime="0-0-0", value of tianmu_type.attrtype is common::ColumnType::DATETIME and buf->x is 0.
So it return 1.

bool Item_tianmufield::get_time(MYSQL_TIME *ltime) {
  if ((null_value = buf->null) ||
      ((tianmu_type.attrtype == common::ColumnType::DATETIME || tianmu_type.attrtype == common::ColumnType::DATE) &&
       buf->x == 0))  // zero date is illegal
    return 1;         // like in Item_field::get_time - return 1 on null value.
  FeedValue();
  return ivalue->get_time(ltime);
}

Then it set null_value = 1.

inline bool get_arg0_time(MYSQL_TIME *ltime) {
    return (null_value = args[0]->get_time(ltime));
  }

So it return a nullptr in val_str() function in item_timefunc.