fix: ban empty anonymous nodes with diagnostic

[zharinov]

Summary

• Add EmptyAnonymousNode diagnostic for "" and '' in queries
• Error message: "empty anonymous node" with hint "use a valid anonymous node or remove it"
• Empty strings still parse to valid CST (error recovery) and compile to wildcard behavior in IR

Why

Empty anonymous nodes like "" are almost certainly user errors — they match nothing useful. This adds a clear diagnostic to guide users toward valid syntax.