Fix T63407: 'Select linked' operator in UV editor is broken in when multi-object-editing contains an empty mesh

Since BM_uv_vert_map_create would return NULL for an empty mesh, code
would then return from uv_select_linked_multi [where it should just skip
and continue instead...]

Maniphest Tasks: T63407

Differential Revision: https://developer.blender.org/D6441

source/blender/editors/uvedit/uvedit_ops.c

@@ -1286,7 +1286,7 @@ static void uv_select_linked_multi(Scene *scene,
     vmap = BM_uv_vert_map_create(em->bm, limit, !select_faces, false);
 
     if (vmap == NULL) {
-      return;
+      continue;
     }
 
     stack = MEM_mallocN(sizeof(*stack) * (em->bm->totface + 1), "UvLinkStack");