trie: stop indexing empty directory nodes

There is a strange bug in trie that seems related to indexing empty directory nodes. The actual root cause
is still unknown to me and I fail to reproduce the issue. In our production, we notice the issue associates
with a cron job for removing files (which would causes the generation of empty directories).

Considering that empty directories doesnt produce much value in go-carbon/graphite, excluding it from
index should cause no harm. And this change also appears to resolve the issue on our production.

carbonserver/carbonserver.go

@@ -990,21 +990,27 @@ func (listener *CarbonserverListener) updateFileList(dir string, cacheMetricName
 					delete(cacheMetricNames, trimmedName)
 				} else {
 					if listener.trieIndex {
-						var dataPoints int64
-						if isFullMetric && listener.estimateSize != nil {
-							m := strings.ReplaceAll(trimmedName, "/", ".")
-							m = m[1 : len(m)-4]
-							_, dataPoints = listener.estimateSize(m)
-						}
-
-						var physicalSize = info.Size()
-						if stat, ok := info.Sys().(*syscall.Stat_t); ok {
-							physicalSize = stat.Blocks * 512
-						}
-
-						if err := trieIdx.insert(trimmedName, info.Size(), physicalSize, dataPoints); err != nil {
-							// It's better to just log an error than stop indexing
-							listener.logTrieInsertError(logger, "updateFileList.trie: failed to index path", trimmedName, err)
+						// WHY:
+						//   * this would only affects empty directories
+						//   * indexing empty directories causes an strange bug in trie index
+						//   * empty dir isn't useful (at least most of the time)?
+						if isFullMetric {
+							var dataPoints int64
+							if listener.estimateSize != nil {
+								m := strings.ReplaceAll(trimmedName, "/", ".")
+								m = m[1 : len(m)-4]
+								_, dataPoints = listener.estimateSize(m)
+							}
+
+							var physicalSize = info.Size()
+							if stat, ok := info.Sys().(*syscall.Stat_t); ok {
+								physicalSize = stat.Blocks * 512
+							}
+
+							if err := trieIdx.insert(trimmedName, info.Size(), physicalSize, dataPoints); err != nil {
+								// It's better to just log an error than stop indexing
+								listener.logTrieInsertError(logger, "updateFileList.trie: failed to index path", trimmedName, err)
+							}
 						}
 					} else {
 						files = append(files, trimmedName)