Fix yaml-to-dhall support for empty objects (#1186)

* Fix `yaml-to-dhall` support for empty objects

`yaml-to-dhall` was decoding an empty object into:

```dhall
[] : { mapKey : Text, mapValue : Text }
```

... instead of:

```dhall
[] : List { mapKey : Text, mapValue : Text }
```

... which this change fixes

* Add missing test files

* Hopefully fix test failure on Windows

I suspect that the test failure is due to the locale for the test suite
not being set to UTF8

dhall-json/src/Dhall/JSONToDhall.hs

@@ -534,7 +534,7 @@ dhallFromJSON (Conversion {..}) expressionType =
 
                       elementType
                           | null elements =
-                              Just (D.Record [ ("mapKey", D.Text), ("mapValue", "JSON") ])
+                              Just (D.App D.List (D.Record [ ("mapKey", D.Text), ("mapValue", "JSON") ]))
                           | otherwise =
                               Nothing
 

dhall-json/tasty/Main.hs

@@ -19,11 +19,15 @@ import qualified Dhall.JSONToDhall    as JSONToDhall
 import qualified Dhall.Parser
 import qualified Dhall.TypeCheck
 import qualified Dhall.Yaml
+import qualified GHC.IO.Encoding
 import qualified Test.Tasty
 import qualified Test.Tasty.HUnit
 
 main :: IO ()
-main = Test.Tasty.defaultMain testTree
+main = do
+    GHC.IO.Encoding.setLocaleEncoding GHC.IO.Encoding.utf8
+
+    Test.Tasty.defaultMain testTree
 
 testTree :: TestTree
 testTree =
@@ -36,6 +40,7 @@ testTree =
             (Dhall.Yaml.defaultOptions { Dhall.Yaml.quoted = True })
             "./tasty/data/quoted"
         , testJSONToDhall "./tasty/data/emptyAlternative"
+        , testJSONToDhall "./tasty/data/emptyObject"
         , Test.Tasty.testGroup "Nesting"
             [ testDhallToJSON "./tasty/data/nesting0"
             , testDhallToJSON "./tasty/data/nesting1"

dhall-json/tasty/data/emptyObject.dhall

@@ -0,0 +1,17 @@
+  λ(JSON : Type)
+→ λ ( json
+    : { array :
+          List JSON → JSON
+      , bool :
+          Bool → JSON
+      , null :
+          JSON
+      , number :
+          Double → JSON
+      , object :
+          List { mapKey : Text, mapValue : JSON } → JSON
+      , string :
+          Text → JSON
+      }
+    )
+→ json.object ([] : List { mapKey : Text, mapValue : JSON })

dhall-json/tasty/data/emptyObject.json

@@ -0,0 +1 @@
+{}

dhall-json/tasty/data/emptyObjectSchema.dhall

@@ -0,0 +1,17 @@
+  ∀(JSON : Type)
+→ ∀ ( json
+    : { array :
+          List JSON → JSON
+      , bool :
+          Bool → JSON
+      , null :
+          JSON
+      , number :
+          Double → JSON
+      , object :
+          List { mapKey : Text, mapValue : JSON } → JSON
+      , string :
+          Text → JSON
+      }
+    )
+→ JSON