fix(ui): empty line before the next message after :silent command

Problem:
  The next command after `silent !{cmd}` or `silent lua print('str')`
  prints an empty line before printing a message, because these commands
  set `msg_didout = true` despite not printing any messages.

Solution:
  Set `msg_didout = true` only if `msg_silent == 0`

src/nvim/lua/executor.c

@@ -955,7 +955,9 @@ static void nlua_print_event(void **argv)
       break;
     }
     msg(str + start, 0);
-    msg_didout = true;  // Make blank lines work properly
+    if (msg_silent == 0) {
+      msg_didout = true;  // Make blank lines work properly
+    }
   }
   if (len && str[len - 1] == NUL) {  // Last was newline
     msg("", 0);