[postgres] fix doc_count to be 0 by default

devops/postgres/schema.sql

@@ -164,7 +164,7 @@ CREATE TABLE public.context_node_ngrams (
     ngrams_id     INTEGER NOT NULL REFERENCES public.ngrams   (id) ON DELETE CASCADE,
     ngrams_type   INTEGER  ,
     weight double precision,
-    doc_count     INTEGER,
+    doc_count     INTEGER DEFAULT 0,
     PRIMARY KEY (context_id, node_id, ngrams_id, ngrams_type)
   );
 ALTER TABLE public.context_node_ngrams OWNER TO gargantua;

devops/postgres/upgrade/0.0.6.7.3.sql

 ALTER TABLE context_node_ngrams
-  ADD COLUMN doc_count INTEGER;
+  ADD COLUMN doc_count INTEGER DEFAULT 0;