Fix various Expr cloning bugs

When `Expr.clone()` is called, various errors(NPE/TypeCastingError/recursive/functional error) are occurs. This issue should be solved for others issues reusing `Expr`. Please refer the following code.

• NPE case (Aggregation.java, UnaryOperator.java, CreateTable, AlterTable)

  -    aggregation.namedExprs = new NamedExpr[namedExprs.length];
  -    for (int i = 0; i < namedExprs.length; i++) {
  -      aggregation.namedExprs[i] = (NamedExpr) namedExprs[i].clone();
  +    if (namedExprs != null) {
  +      aggregation.namedExprs = new NamedExpr[namedExprs.length];
  +      for (int i = 0; i < namedExprs.length; i++) {
  +        aggregation.namedExprs[i] = (NamedExpr) namedExprs[i].clone();
  +      }
       }
   
  -    aggregation.groups = new GroupElement[groups.length];
  -    for (int i = 0; i < groups.length; i++) {
  -      aggregation.groups[i] = (GroupElement) groups[i].clone();
  +    if (groups != null) {
  +      aggregation.groups = new GroupElement[groups.length];
  +      for (int i = 0; i < groups.length; i++) {
  +        aggregation.groups[i] = (GroupElement) groups[i].clone();
  +      }
       }
  

-    alter.addNewColumn = (ColumnDefinition) addNewColumn.clone();
+    if (addNewColumn != null) {
+      alter.addNewColumn = (ColumnDefinition) addNewColumn.clone();
+    }

• Recursive (BetweenPredicate.java)

  -    between.begin = (Expr) between.clone();
  +    between.begin = (Expr) begin.clone();
  

• Type Casting Error (ValueListExpr.java)

  -      valueListExpr.values = (Expr[]) values[i].clone();
  +      valueListExpr.values[i] = (Expr) values[i].clone();
  

• Functional Error (CreateTable.java)

  -      if (range.specifiers != null) {
  +      if (specifiers != null) {