Center "Why not start the conversation" for PMs.

(imported from commit ed01d464719b76a8c52c8b205395cc853077a1e1)

static/styles/zulip.css

@@ -2536,6 +2536,7 @@ div.floating_recipient {
     text-align: center;
 }
 
+#empty_narrow_private_message p,
 #empty_narrow_message p {
     text-align: center;
 }