CALCULATETABLE VALUES with filter

Hello Power BI Folks,

I created a calculated table bases on the folowing DAX and this DAX still works :-)

Workschedule = SUMMARIZE( GENERATE( M, CALCULATETABLE (VALUES( 'Calendar'[Date]),DATESBETWEEN('Calendar'[Date],'M'[scheduledStart],'M'[scheduledEnd]))),'Calendar'[Date],M[SystemUserId],M[ActivityId],M[FullName],M[ContractUrenPerWeek],M[Department],M[BusinessUnit],M[Subject],M[Description])

The result of this working DAX is a table with sometimes  rows with ActivityId is empy, and sometimes ActivityId contains a value. 

Now I need a (new?) calculated table with only row values with M[ActivityId]=BLANK(). Based on this new calculated table with a filter I'm able to create an essential relationship. (filter out in visual, page level or report level is not possible). 

Any ideas? Help is greatly appreciated!