Fix for empty arrays constructed when internalising reduce streams.
Fix for empty arrays constructed when internalising reduce streams.

ihc/futhark,diku-dk/futhark,HIPERFIT/futhark,ihc/futhark,diku-dk/futhark,ihc/futhark,diku-dk/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark