Fix bug in flash backup rate limiter (#880)

Don't try to read from an empty file

plugin/source/dynamix.unraid.net/usr/local/emhttp/plugins/dynamix.my.servers/include/UpdateFlashBackup.php

@@ -126,7 +126,7 @@ function set_git_config($name, $value) {
 
 function readFromFile($file): string {
   $text = "";
-  if (file_exists($file)) {
+  if (file_exists($file) && filesize($file) > 0) {
       $fp = fopen($file,"r");
       if (flock($fp, LOCK_EX)) {
           $text = fread($fp, filesize($file));