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) { \

src/nvim/ex_cmds.c

@@ -1346,7 +1346,9 @@ void do_shell(char *cmd, int flags)
   // 1" command to the terminal.
   ui_cursor_goto(msg_row, msg_col);
   (void)call_shell(cmd, (ShellOpts)flags, NULL);
-  msg_didout = true;
+  if (msg_silent == 0) {
+    msg_didout = true;
+  }
   did_check_timestamps = false;
   need_check_timestamps = true;
 

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);