robustness improvement: rewind file before writing empty JSON skeleton

prevents creating corrupted JSON
if the previous file content was ``{}``, the result would be ``{"0":{}}`` if the file position is not reset first.

wled00/file.cpp

@@ -226,6 +226,7 @@ bool appendObjectToFile(const char* key, JsonDocument* content, uint32_t s, uint
   if (f.size() < 4) { // file uninitialized -> write minimal skeleton
     char init[12];
     strcpy_P(init, PSTR("{\"0\":{}}"));
+    f.seek(0, SeekSet);           // rewind to ensure we overwrite from the start
     f.print(init);
   }