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/api/vim.c

@@ -1744,7 +1744,9 @@ static void write_msg(String message, bool to_err, bool writeln)
     } else { \
       msg(line_buf->items, 0); \
     } \
-    msg_didout = true; \
+    if (msg_silent == 0) { \
+      msg_didout = true; \
+    } \
     kv_drop(*line_buf, kv_size(*line_buf)); \
     kv_resize(*line_buf, LINE_BUFFER_MIN_SIZE); \
   } else if (c == NUL) { \