Add a warning when a component is empty

[DanChaltiel]

In the NIH example sample, the AAPC output is empty.
I don't know why this happens, but this should throw a warning.

library(tidyverse)
library(nih.joinpoint)
options(joinpoint_path="path/jpCommand.exe")
run_opt = run_options(model="ln", max_joinpoints=3, n_cores=3)
jp = joinpoint(nih_sample_data, x=year, y=rate, by=sex, se=se, run_opts=run_opt)
jp %>% map_dbl(nrow)
#          aapc            apc    data_export selected_model      perm_test         report 
#             0             12            105              3              9             12