I want xref-find-definitions that uses the second backend when the first backend doesn't find anything

Question

I program in Haskell using lsp-mode with lsp-haskell. I need to have a working "jump to definition". xref-find-definitions using lsp--xref-backend sometimes fails to find the definition. xref-find-definitions using etags--xref-backend usually finds the definition together with other unrelated definitions. I want to have a command that runs lsp--xref-backend first, and if it doesn't find anything, runs etags--xref-backend. How do I achieve that? Simply setting xref-backend-functions to '(lsp--xref-backend etags--xref-backend) never invokes the etags backend even when the lsp backend doesn't find anything.