bool = pcoverage:IsNull()
True if the coverage is NULL, i.e. invalid. If functions
that allocate objects fail, they might not throw an error but simply set the
object to NULL. You can use this function to check if object allocation has
failed in which case the coverage will be NULL.
Also, certain getter functions can return a NULL object in case the object
doesn't exist. You can use this function to check if a getter function returned
a NULL handle.
True if the coverage is NULL, otherwise False