Jetpack: Stats: Fix csv trailing newline handling (#39787)

When #39665 switched from incorrectly using `str_getcsv( $csv, "\n" )`
to `explode()`, a minor difference between the two was unnoticed: the
former does not produce an empty element at the end of the result on a
trailing newline, while the latter does.

Add an `rtrim` on the data to ensure there isn't a trailing newline.

projects/plugins/jetpack/changelog/fix-stats-explode-trailing-newline

@@ -0,0 +1,5 @@
+Significance: patch
+Type: other
+Comment: Fix bug in #39665 related to a trailing newline in the CSV data.
+
+

projects/plugins/jetpack/modules/stats.php

@@ -1668,7 +1668,7 @@ function stats_get_remote_csv( $url ) {
  */
 function stats_str_getcsv( $csv ) {
 	// @todo Correctly handle embedded newlines. Note, despite claims online, `str_getcsv( $csv, "\n" )` does not actually work.
-	$lines = explode( "\n", $csv );
+	$lines = explode( "\n", rtrim( $csv, "\n" ) );
 	return array_map(
 		function ( $line ) {
 			// @todo When we drop support for PHP <7.4, consider passing empty-string for `$escape` here for better spec compatibility.