MDL-47065 notes: prevent empty spaces added as note

(And updated behat test for this test case)

user/addnote.php

@@ -55,11 +55,12 @@ if (!empty($users) && confirm_sesskey()) {
     $note->format = FORMAT_PLAIN;
     foreach ($users as $k => $v) {
         $user = $DB->get_record('user', array('id' => $v));
-        if (!$user || empty($contents[$k])) {
+        $content = trim($contents[$k]);
+        if (!$user || empty($content)) {
             continue;
         }
         $note->id = 0;
-        $note->content = $contents[$k];
+        $note->content = $content;
         $note->publishstate = $states[$k];
         $note->userid = $v;
         note_save($note);