Don't call fwrite() with len == 0 when writing out relcache init file.

Noticed via -fsanitize=undefined.

Backpatch to all branches, for the same reasons as 46ab07ffda9.

Discussion: https://postgr.es/m/20220323173537.ll7klrglnp4gn2um@alap3.anarazel.de
Backpatch: 10-

src/backend/utils/cache/relcache.c

@@ -6485,7 +6485,7 @@ write_item(const void *data, Size len, FILE *fp)
 {
 	if (fwrite(&len, 1, sizeof(len), fp) != sizeof(len))
 		elog(FATAL, "could not write init file");
-	if (fwrite(data, 1, len, fp) != len)
+	if (len > 0 && fwrite(data, 1, len, fp) != len)
 		elog(FATAL, "could not write init file");
 }