format_appendix: replace empty coverage table with note

External scorers are responsible for providing the *.coverage.json,
and that file can have an overall coverage score but not per file
scores.  If there are no rows in the coverage table, replace the table
with a note to make it clear that this is expected rather than a
rendering error.

Thanks to @seth127 for the suggestion.

Re: #70 (comment)

R/format-report.R

@@ -861,6 +861,8 @@ format_appendix <- function(extra_notes_data, return_vals = FALSE, scorecard_typ
       } else {
         stop("Unknown error type: ", err_type)
       }
+    } else if (nrow(cov_results_df) == 0) {
+      cat("Per file test coverage not provided.")
     } else {
       cat(knitr::knit_print(cov_results_flex))
     }

tests/testthat/test-format-report-external.R

@@ -66,4 +66,22 @@ describe("formatting functions with external scores", {
       "check\noutput\n"
     )
   })
+
+  it("format_appendix: no per file coverage", {
+    results_dir <- local_create_external_results()
+    covfile <- get_result_path(results_dir, "coverage.json")
+    cov <- jsonlite::read_json(covfile)
+    cov[["files"]] <- list()
+    jsonlite::write_json(cov, covfile)
+
+    expect_output(
+      format_appendix(
+        list(
+          check_output = "anything",
+          cov_results_df = read_coverage_results(results_dir)
+        )
+      ),
+      "Per file test coverage not provided"
+    )
+  })
 })