Merge Query joins WhenNodes with comma which causes a syntax error

[gittgott]

When you have multiple whens within a merge query, the WhenNodes are joined with a comma. A minimal example of this can be seen in the following kysely playground: https://kyse.link/A9Ndr

The affected lines are as follows (with the comma after @1):

UPDATE SET
  "first_name" = @1,
WHEN NOT MATCHED THEN

I know that this causes a syntax error within mssql, but am not as sure about the other dialects that kysely includes.

I'll make a draft PR shortly in attempt to fix this.