Fix clock-in failing when no client is selected

clockIn and create used `input.clientId?.trim() ?? null`, but clients
always send `""` (not undefined) for no-client. `??` doesn't catch the
empty string, so `""` was inserted into time_entries.client_id,
violating the foreign key to clients.id. Use `|| null` to normalize a
blank string to null, matching updateRunning.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/server/api/routers/time-entries.ts

@@ -242,7 +242,7 @@ export const timeEntriesRouter = createTRPCRouter({
         });
       }
 
-      const clientId = input.clientId?.trim() ?? null;
+      const clientId = input.clientId?.trim() || null;
       let clientRecord: { defaultHourlyRate: number | null } | null = null;
       if (clientId) {
         const found = await ctx.db.query.clients.findFirst({
@@ -514,7 +514,7 @@ export const timeEntriesRouter = createTRPCRouter({
   create: protectedProcedure
     .input(createSchema)
     .mutation(async ({ ctx, input }) => {
-      const clientId = input.clientId?.trim() ?? null;
+      const clientId = input.clientId?.trim() || null;
       if (clientId) {
         const client = await ctx.db.query.clients.findFirst({
           where: and(eq(clients.id, clientId), eq(clients.createdById, ctx.session.user.id)),