🐛 Fix subscribe to undefined stream error in use-stream hook (#8633)

Add a nil guard before subscribing to the stream in the use-stream
hook. When a nil/undefined stream is passed (e.g., from a conditional
expression or timing edge case during React rendering), the subscribe
call on undefined causes a TypeError. The guard ensures we only
subscribe when the stream is defined.

Signed-off-by: Andrey Antukh <niwi@niwi.nz>

frontend/src/app/main/ui/hooks.cljs

@@ -214,10 +214,11 @@
    (mf/use-effect
     deps
     (fn []
-      (let [sub (->> stream (rx/subs! on-subscribe))]
-        #(do
-           (rx/dispose! sub)
-           (when on-dispose (on-dispose))))))))
+      (when stream
+        (let [sub (->> stream (rx/subs! on-subscribe))]
+          #(do
+             (rx/dispose! sub)
+             (when on-dispose (on-dispose)))))))))
 
 ;; https://reactjs.org/docs/hooks-faq.html#how-to-get-the-previous-props-or-state
 ;; FIXME: replace with rumext