Nulls and the MERGE statement: I need to set a value to infinity. How?

You can change the ON part of the merge statement, putting in a check for when both source and target are null.

MERGE tgt
USING src
ON ( -- enter non-nullable columns to match on ...
    tgt.A = src.A
    AND (tgt.C = src.C OR (tgt.C IS NULL AND src.C IS NULL))
)
WHEN MATCHED -- ...

You can use

WHEN MATCHED AND EXISTS (SELECT tgt.C EXCEPT SELECT src.C)

See this article for more on this issue.