orphan checker name element in bug filter
In Codechecker Run Overview, checker filter dropdown, there is an orphan elment "(1)"  under the Unspecified checkers list.

clicking on it, lists all results.

Reproducible by checking 3.1.2
http://md-mtas2.tsp.eth.ericsson.se:9997/#run=2

diff --git a/storage_server/report_server.py b/storage_server/report_server.py
index 0ea36638..a26058ea 100644
--- a/storage_server/report_server.py
+++ b/storage_server/report_server.py
@@ -417,6 +417,8 @@ class CheckerReportHandler(object):
             action = self.session.query(BuildAction).get(build_action_id)
             assert action is not None
 
+            checker_id = checker_id or 'NOT FOUND'
+
             # TODO: perfomance issues when executing the following query on
             # large databaseses?
             reports = self.session.query(self.report_ident) \\
['storage_server/report_server.py']