[win32_event_log] Fix small regression on `msg_text` selection
[win32_event_log] Fix small regression on `msg_text` selection

When the windows event `Message` evaluates to a "false" value, do not
select its value as the event's `msg_text`.

Same behavior for the `InsertionStrings`.

Fixes the integration test.

cberry777/dd-agent,manolama/dd-agent,cberry777/dd-agent,manolama/dd-agent,takus/dd-agent,takus/dd-agent,takus/dd-agent,cberry777/dd-agent,indeedops/dd-agent,Wattpad/dd-agent,Wattpad/dd-agent,takus/dd-agent,jyogi/purvar-agent,takus/dd-agent,cberry777/dd-agent,cberry777/dd-agent,indeedops/dd-agent,indeedops/dd-agent,manolama/dd-agent,Wattpad/dd-agent,jyogi/purvar-agent,jyogi/purvar-agent,Wattpad/dd-agent,jyogi/purvar-agent,Wattpad/dd-agent,indeedops/dd-agent,indeedops/dd-agent,jyogi/purvar-agent,manolama/dd-agent,manolama/dd-agent