Yes actually that is behavior I have seen as well when an aggregate is used in the query.

In order to eliminate the NULL values you will need to group the data. If a GROUP BY is not used or check for NULL is not in where condition and will get unexpected results.

select p.paynum, c.gcamount from #payment p

cross APPLY(

SELECT SUM(gcamount) gcamount

FROM #certificate c

WHERE c.paynum = p.paynum

GROUP BY c.paynum

)c