Fix regression with empty list arguments in function application

Fixes #268

src/Nixfmt/Pretty.hs

@@ -413,7 +413,7 @@ prettyApp indentFunction pre hasPost f a =
               <> pretty parclose
         where
           -- If the brackets are on different lines, keep them like that
-          sur = if sourceLine paropen /= sourceLine parclose then hardline else line
+          sur = if sourceLine paropen /= sourceLine parclose then hardline else if length (unItems items) > 0 then line else hardspace
       absorbInner expr = pretty expr
 
       -- Render the last argument of a function call

test/diff/apply_with_lists/in.nix

@@ -75,4 +75,7 @@
       out = "20170512";
     }
   ] null)
+  # https://github.com/NixOS/nixfmt/issues/268 regression test. [] and {} should be treated the same
+  (defaultNullOpts.mkNullable (with types; either str (listOf str)) [] "Some example long text that causes the line to be too long.")
+  (defaultNullOpts.mkNullable (with types; either str (listOf str)) {} "Some example long text that causes the line to be too long.")
 ]

test/diff/apply_with_lists/out-pure.nix

@@ -107,4 +107,11 @@
     ]
     null
   )
+  # https://github.com/NixOS/nixfmt/issues/268 regression test. [] and {} should be treated the same
+  (defaultNullOpts.mkNullable (
+    with types; either str (listOf str)
+  ) [ ] "Some example long text that causes the line to be too long.")
+  (defaultNullOpts.mkNullable (
+    with types; either str (listOf str)
+  ) { } "Some example long text that causes the line to be too long.")
 ]

test/diff/apply_with_lists/out.nix

@@ -117,4 +117,11 @@
     ]
     null
   )
+  # https://github.com/NixOS/nixfmt/issues/268 regression test. [] and {} should be treated the same
+  (defaultNullOpts.mkNullable (
+    with types; either str (listOf str)
+  ) [ ] "Some example long text that causes the line to be too long.")
+  (defaultNullOpts.mkNullable (
+    with types; either str (listOf str)
+  ) { } "Some example long text that causes the line to be too long.")
 ]