nil是空指针
Null是函数，下面是帮助文件的内容
function Null: Variant;
Description
Use Null to obtain a Null variant that can indicate unknown or missing data. Null Variants can be assigned to variant variables in an application that must contain a null value. Assigning Null to a variant variable does not cause an error, and Null can be returned from any function with a variant return value.
Assigning Null to a variable of any type other than Variant causes either a compile-time error or raises an EVariantTypeMismatch exception. For example, in the following code the assignment of v, the Null variant, to variant q is successful
whereas the conversion of variant v, which is now Null, to the integer return type of the Test function, raises an exception.
