Improve handling of optional amount fields in transaction forms

Update the `avoirSchema` in `Avoirs.tsx` to correctly preprocess empty string values for the `amount` field, ensuring it's treated as undefined.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 9fcf4b21-eb0c-4e53-a567-e2ce4a6ad869
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/9fcf4b21-eb0c-4e53-a567-e2ce4a6ad869/KZQgDPI

client/src/pages/Avoirs.tsx

@@ -71,7 +71,7 @@ const avoirSchema = z.object({
   supplierId: z.coerce.number().min(1, "Veuillez sélectionner un fournisseur"),
   groupId: z.coerce.number().min(1, "Veuillez sélectionner un magasin"),
   invoiceReference: z.string().optional(),
-  amount: z.coerce.number().positive().optional(),
+  amount: z.preprocess((val) => val === "" ? undefined : val, z.coerce.number().positive().optional()),
   comment: z.string().optional(),
   commercialProcessed: z.boolean().optional(),
   status: z.enum(["En attente de demande", "Demandé", "Reçu"]).optional(),