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.

---------

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

wled00/presets.cpp

@@ -123,7 +123,15 @@ bool getPresetName(byte index, String& name)
 
 void initPresetsFile()
 {
-  if (WLED_FS.exists(getFileName())) return;
+  if (WLED_FS.exists(getFileName())) {
+    // treat an empty JSON file (e.g. "{}", "{ }") the same as a missing file:
+    // f.size() < 4 is the same threshold used in appendObjectToFile() to detect an uninitialized file
+    File f = WLED_FS.open(getFileName(), "r");
+    bool empty = f && f.size() < 4;             // file does exist due to previous "if"
+    if (f) f.close();
+    if (empty) WLED_FS.remove(getFileName());   // remove the empty file so it can be recreated below
+    else return;                                // file not empty -> keep (nothing to init)
+  }
 
   StaticJsonDocument<64> doc;
   JsonObject sObj = doc.to<JsonObject>();