IsNull (geometry Data Type)
Specifies whether the geometry instance is null. Returns 'NULL' if the instance is null; returns 0 if the instance is not null.
IsNull can be used to test whether a geometry instance is null. This can produce somewhat confusing results, returning 0 if the instance is not null, but null if the instance is null.
This method is primarily used by the SQL Server infrastructure; we recommend against using IsNull to test whether an instance is null.