🐛(frontend) stop init_virtual_background from firing on blur updates

A regression introduced with the custom virtual background feature
(16daf7b8, March 26): every blur init on Firefox, and every update
call, was going through `init_virtual_background`, which raised an
error because there is no virtual background to initialize in that
code path.

The existing guard was incorrect. Replace it with an early return,
so `init_virtual_background` is only executed when there is actually
a virtual background to initialize.

This does not fix the other virtual background issues that currently
prevent some users from activating their camera; those will be
addressed separately.

src/frontend/src/features/rooms/livekit/components/blur/BackgroundCustomProcessor.ts

@@ -105,9 +105,7 @@ export class BackgroundCustomProcessor implements BackgroundProcessorInterface {
 
   _initVirtualBackgroundImage() {
     if (this.options.type !== 'virtual') {
-      throw new Error(
-        'Virtual background is only supported for virtual background'
-      )
+      return
     }
 
     const needsUpdate =