Fix robustness issue: empty JSON {} in cfg.json or wsec.json (#355)

* Treat empty or effectively-empty configuration files as invalid: restore defaults, optionally fall back to stored settings, and avoid silently using blank configurations.
* Improve presets handling by detecting and recreating empty or undersized preset files so presets are reliably initialized and usable.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: softhack007 <91616163+softhack007@users.noreply.github.com>

wled00/file.cpp

@@ -223,7 +223,7 @@ bool appendObjectToFile(const char* key, JsonDocument* content, uint32_t s, uint
   uint32_t pos = 0;
   if (!f) return false;
 
-  if (f.size() < 3) {
+  if (f.size() < 4) { // file uninitialized -> write minimal skeleton
     char init[12];
     strcpy_P(init, PSTR("{\"0\":{}}"));
     f.print(init);