Thanks to all. Here are my test results:
SELECT COUNT(*) as CNT_TOT,
(SELECT count(VAR1) from table_name where VAR1 = '""') as CNT1_MISSING
This one works.
SELECT COUNT(*) as CNT_TOT,
SUM(IF var1 IS NULL 1 ELSE 0) as CNT1_MISSING
For this one, the syntax is not recognized. It works in SQL server, not in HANA PASS THROUGH.