Adapt R configuration sanity checks to revision range generation

db3de9f8bd3 introduced automatic means for revision range generation,
but this is not yet known to the R configuration parser, which
exits with an error if no explicit ranges are specified. Honour
the update.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>
Handle empty clusters in the cluster analysis stage

For small projects, it can happen that certain revision ranges
contain only a single contributor, leading to pathological
collaboration graphs. This case is handled partially
in 92fd49b, but some problems in the R code remain. Fix
them by ignoring empty clusters.

Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@siemens.com>