hello-world-ui-test: specify empty library due to cabal bug

Since we are making the Haskell code just to test a Purescript project,
we have ended up with no library or executable in the cabal file, which
means we have to specify a null one.

haskell/cabal#6750

offchain/hello-world-ui/test/selenium/hello-world-ui-selenium-test.cabal

@@ -2,6 +2,11 @@ cabal-version: 2.4
 name:          hello-world-ui-selenium-test
 version:       0.1.0.0
 
+-- Cabal bug requires us to specify empty library for haskell.nix
+-- https://github.com/haskell/cabal/issues/6750
+-- https://github.com/input-output-hk/haskell.nix/blob/ebdc0030e6973629c1ed7dba53c1b1d154c3e75e/builder/comp-builder.nix#L264
+library
+
 test-suite sydtest-webdriver
   default-language: Haskell2010
   type:             exitcode-stdio-1.0