With the release of Tableau 8.1, the ability to "Show Empty Rows" (or columns) was removed when you use a secondary data source. This functionality was available in versions 8.0 and 7.0 but was disabled (greyed out) in 8.1 because it can cause Tableau to unexpectedly quit. This is the response I received from Tableau Support:
"The "Show Empty Columns" and "Show Empty Rows" menu options are intentionally disabled in Tableau Desktop 8.1 when secondary dimensions from a blended data source are included in a view as there are certain conditions in which the secondary dimensions can cause Tableau Desktop to quit unexpectedly.
The feature was not originally designed to handle secondary dimensions, however this design may change in the future. If you feel that there is an important use case for the feature, I would suggest posting it to our community Ideas board"
This idea is therefore for this functionality to be reintroduced.
See attached workbook for example -- Show missing values works correctly in v8 but opening in v8.1 will not produce the same result and the option is greyed out