The fastest way to count null values

select sum(case when a is null then 1 else 0 end) count_nulls
     , count(a) count_not_nulls 
  from <YourTable>;

declare @DefaultDataPath  sql_variant
declare @DefaultLogPath  sql_variant

set @DefaultDataPath = SERVERPROPERTY('InstanceDefaultDataPath')
set @DefaultLogPath = SERVERPROPERTY('InstanceDefaultLogPath')

select @DefaultDataPath, @DefaultLogPath