Technically, that's not an empty string. It might contain a blank character like CHAR(0).
Here's an example:

WITH UD_Indiv_Data AS(
  SELECT '' YAB UNION ALL
  SELECT ' ' YAB UNION ALL
  SELECT '20170115' YAB UNION ALL
  SELECT '156431564' YAB UNION ALL
  SELECT '20171301' YAB UNION ALL
  SELECT '20171211 12:23' YAB UNION ALL
  SELECT CHAR(0)
)
SELECT YAB,
  ASCII(YAB) AS FirstCharASCII,
  TRY_CAST( YAB AS DATETIME) AS DatetimeYAB
FROM UD_Indiv_Data
--WHERE YAB IS NOT NULL
--AND TRY_CAST( YAB AS DATETIME) IS NULL;