LibraryExecutionError if function returns empty data.frame()

A webservice returning an empty array fails with "Module execution encountered an internal library error."

```R
df <- data.frame(x=1,y=2)
func <- function(df) {
  return(data.frame())
}
service <- publishWebService(ws.priv,func,"testService",inputSchema = df)
consume(service,df)
```