Include null detection in AST NodeEnumToString func #30 Fix

[daeisbae]

Changes

• When debugging Null expression , it showed null expression as InvalidExpression. #30 Fix

Previous

./AParser 
>>> 1 + null
ProgramStatement {
BinaryExpression (Left Value : IntegerExpression (Value : 1), Op Value : +, Right Value : InvalidExpression ( Value : 'NULL' ), )
}
>>> null
ProgramStatement {
InvalidExpression ( Value : 'NULL' )
}

Current

./AParser 
>>> null
ProgramStatement {
NullExpression ( Value : 'NULL' )
}
>>> 1 + 3 + null
ProgramStatement {
BinaryExpression (Left Value : BinaryExpression (Left Value : IntegerExpression (Value : 1), Op Value : +, Right Value : IntegerExpression (Value : 3), ), Op Value : +, Right Value : NullExpression ( Value : 'NULL' ), )
}