fix ABI check for uninstalled package

(which may be redundant now)

ghc-xhtml is not built for bootstrap, so check was failing

ghc.spec

@@ -422,6 +422,7 @@ make test
 echo "Checking package ABI hashes:"
 for i in %{ghc_packages_list}; do
   old=$(ghc-pkg field $i id --simple-output)
+  if [ -n "$old" ]; then
     new=$(/usr/lib/rpm/ghc-pkg-wrapper %{buildroot}%{ghclibdir} field $i id --simple-output)
     if [ "$old" != "$new" ]; then
       echo "ABI hash for $i changed!:" >&2
@@ -430,6 +431,9 @@ for i in %{ghc_packages_list}; do
     else
       echo "($old unchanged)"
     fi
+  else
+    echo "($i not installed)"
+  fi
 done
 if [ "$ghc_abi_hash_change" = "yes" ]; then
    echo "ghc ABI hash change: aborting build!" >&2