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>

Author: Copilot

wled00/cfg.cpp

@@ -659,7 +659,7 @@ void deserializeConfigFromFS() {
   DEBUG_PRINTLN(F("Reading settings from /cfg.json..."));
 
   success = readObjectFromFile("/cfg.json", nullptr, &doc);
-  if (!success) { // if file does not exist, optionally try reading from EEPROM and then save defaults to FS
+  if (!success || doc.size() == 0) { // if file does not exist or contains only empty JSON (e.g. "{}"), try reading from EEPROM (if supported) and then save defaults to FS
     releaseJSONBufferLock();
     #ifdef WLED_ADD_EEPROM_SUPPORT
     deEEPSettings();
@@ -1122,7 +1122,7 @@ bool deserializeConfigSec() {
   if (!requestJSONBufferLock(3)) return false;
 
   bool success = readObjectFromFile("/wsec.json", nullptr, &doc);
-  if (!success) {
+  if (!success || doc.size() == 0) { // treat empty JSON (e.g. "{}") the same as a missing file
     releaseJSONBufferLock();
     return false;
   }

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);

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>();