fix: guard iteration picker VirtualList against empty items array (#8273)

When a flow loops over an empty array, the VirtualList component crashes
trying to access index 0 in an empty range. Add a guard to only render
VirtualList when items.length > 0, showing a "No iterations" message
otherwise.

Fixes #8272

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: windmill-internal-app[bot] <windmill-internal-app[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/components/flows/map/FlowJobsMenu.svelte

@@ -142,6 +142,7 @@
 
 					<div class="max-h-[300px]">
 						{#key items}
+							{#if items.length > 0}
 							<VirtualList height={300} width="100%" itemCount={items.length} itemSize={24}>
 								{#snippet header()}{/snippet}
 								{#snippet footer()}{/snippet}
@@ -170,6 +171,9 @@
 									</div>
 								{/snippet}
 							</VirtualList>
+							{:else}
+								<div class="text-xs text-tertiary py-2 px-2">No iterations</div>
+							{/if}
 						{/key}
 
 						<!-- {#each flowJobs ?? [] as id, idx (id)}