Skip merging profraw data if profraw files don't exist and a profdata…

… file already exists (#360)

src/main.rs

@@ -650,6 +650,9 @@ fn merge_profraw(cx: &Context) -> Result<()> {
     .filter_map(Result::ok)
     .collect::<Vec<_>>();
     if profraw_files.is_empty() {
+        if cx.ws.profdata_file.exists() {
+            return Ok(());
+        }
         warn!(
             "not found *.profraw files in {}; this may occur if target directory is accidentally \
              cleared, or running report subcommand without running any tests or binaries",