feat(tvix/eval): warn on empty let-bindings

Change-Id: Ib6ef7ce514abbd3e372dfe9df7137aa36dbda9d4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7770
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/bindings.rs

@@ -647,6 +647,9 @@ impl Compiler<'_> {
                 self.emit_constant(Value::Attrs(Box::new(NixAttrs::empty())), node);
                 return;
             }
+
+            self.emit_warning(node, WarningKind::EmptyLet);
+            return;
         }
 
         // Actually bind values and ensure they are on the stack.