Make statistics_history counters nullable

It's expected that new columns will be added, and these will obviously
have no values in past history entries. For consistency with these,
make existinc columns nullable as well. This also brings schema in sync
to prod.

sql.d/schema/create_schema_tables.sql

@@ -475,10 +475,10 @@ DROP TABLE IF EXISTS statistics_history CASCADE;
 CREATE TABLE statistics_history (
 	ts timestamp with time zone NOT NULL PRIMARY KEY,
 	snapshot jsonb NOT NULL,
-	num_packages integer NOT NULL,
-	num_projects integer NOT NULL,
-	num_problems integer NOT NULL,
-	num_maintainers integer NOT NULL,
+	num_packages integer,
+	num_projects integer,
+	num_problems integer,
+	num_maintainers integer,
 	num_reports_total integer,
 	num_reports_open integer,
 	num_links_total integer,