Fix <field>=* prifix queries in QueryEngine

Sometimes <field>=* prefix queries translate to <field> LIKE '%'
where condition in cassandra. Cassandra doesn't allow such where
clauses and returns error. If QE receives such where clause, ignore
it as it effectively translate to no where condition.

Added systemless test to verify the behavior

Change-Id: I893d3036fd22ce399186f4cc67812de2fa46701c
Closes-bug: #1774364

contrail-opserver/test/utils/analytics_fixture.py

@@ -1316,6 +1316,15 @@ def verify_where_query_prefix(self,generator_obj):
         json_qstr = json.dumps(a_query.__dict__)
         res = vns.post_query_json(json_qstr)
         assert(len(res)>0)
+        a_query = Query(table="FlowSeriesTable",
+                start_time=(generator_obj.session_start_time),
+                end_time=(generator_obj.session_end_time),
+                select_fields=["destvn","SUM(bytes)"],
+                where=[[{"name":"sourcevn","value":"","op":7}]])
+        json_qstr = json.dumps(a_query.__dict__)
+        res = vns.post_query_json(json_qstr)
+        assert(len(res)==2)
+
         return True
 
     def verify_flow_table(self, generator_obj):

contrail-query-engine/where_query.cc

@@ -1534,6 +1534,9 @@ bool WhereQuery::populate_where_vec(AnalyticsQuery *m_query,
         default:
             break;
     }
+    if (val == "%") {
+        return true;
+    }
     GenDb::WhereIndexInfo where_info =
             boost::make_tuple(columnN, db_op, val);
     where_vec->push_back(where_info);