Cleanup: Blenkernel, Clang-Tidy else-after-return fixes

This addresses warnings from Clang-Tidy's `readability-else-after-return`
rule in the `source/blender/blenkernel` module.

No functional changes.

source/blender/blenkernel/intern/undo_system.c

@@ -476,9 +476,8 @@ UndoStep *BKE_undosys_step_push_init_with_type(UndoStack *ustack,
     undosys_stack_validate(ustack, false);
     return us;
   }
-  else {
-    return NULL;
-  }
+
+  return NULL;
 }
 
 UndoStep *BKE_undosys_step_push_init(UndoStack *ustack, bContext *C, const char *name)