zig: prefer .jsUndefined() over .undefined for JSValue (#20332)

src/shell/subproc.zig

@@ -1295,7 +1295,7 @@ pub const PipeReader = struct {
                 return JSC.MarkedArrayBuffer.fromBytes(bytes, bun.default_allocator, .Uint8Array).toNodeBuffer(globalThis);
             },
             else => {
-                return JSC.JSValue.undefined;
+                return .jsUndefined();
             },
         }
     }