[td-4372]<fix>: fix the taosd server crash caused by dropping a child table,...

[td-4372]<fix>: fix the taosd server crash caused by dropping a child table, of which the indexed tag value is null.

src/tsdb/src/tsdbMeta.c

@@ -928,6 +928,11 @@ static int tsdbRemoveTableFromIndex(STsdbMeta *pMeta, STable *pTable) {
   STColumn *pCol = schemaColAt(pSchema, DEFAULT_TAG_INDEX_COLUMN);
 
   char *  key = tdGetKVRowValOfCol(pTable->tagVal, pCol->colId);
+  if (key == NULL) {
+    // treat the column as NULL if we cannot find it
+    key = getNullValue(pCol->type);
+  }
+
   SArray *res = tSkipListGet(pSTable->pIndex, key);
 
   size_t size = taosArrayGetSize(res);