Merging two transactions does not merge categories

STR :

• Consider "mergeable" transactions.
• The "old" transaction has a category, the "new" transaction does not have one.
• Clieck on merge

Expected:

• The kept transaction should have the categoryId kept

Observed:

• The category is "No cagtegory".

This comes from here : https://framagit.org/kresusapp/kresus/-/blob/master/server/models/helpers.ts#L8 the categoryId is now a number. We should check againts "non null" I guess.