guard against missing viewer cache dir
guard against missing viewer cache dir

The viewer cache directory many not yet exist for e.g. newly created
projects, and so attempts to list files in this non-existent directory
will emit an error message.

JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio