Warn if result from filter_*() is empty

[dholstius]

filter_pollutants(), filter_categories(), and filter_facilities() should probably issue a warning if the result they return is empty. This will help prevent, or at least diagnose, crashes when evaluating expressions like:

some_data_with_no_PM2.5 %>% 
  filter_pollutants(
    "PM2.5") %>% 
  chart_annual_emissions()

rlang::is_empty(x) seems like a suitable test to use within the body of filter_*(), just before the result is returned. It returns TRUE if x is NULL, and it also returns TRUE if nrow(x) is a zero-length data frame.

One question is whether the warning should be issued when:

• verbose is FALSE; or
• verbose is not explicitly supplied

... to filter_*().