Suggest you have a look at the keywords "HasData" and "CanShrink", maybe your clue lies there.
Access Help mentiones "HasData" as a means to hide a subreport when it has no data in it. The syntax looks like:
Me!ElementSubRapport.Visible = Me!ElementSubRapport.Report.HasData
See the Help-file for the proper place to put this code.
Apart from this, I often use the CanShrink-property for the control on the main report that holds the subreport, to minimise the subreport when it is empty.