…arning shows correctly (elastic#259199)

Fixes elastic#259169

`createEventSignal` has a bug where it returns incorrect summary results
for pages of source docs that matched no indicators. The calling code
expects `createEventSignal` to return results pertaining only to the
current page, but if no indicators are matched or an error is
encountered, the function instead returns `currentResults` i.e. the sum
of results from all prior pages. The effect is that each time a page
matches no indicators the alert count we track in `createThreatSignals`
_doubles_ because we add `currentResults` to itself.

(cherry picked from commit 22c93a4)