Preserve even weirder Depends behavior

It seems the `Depends` _key_ is indeed required, but a `Null` as the
_value_ is acceptable (and defaulted to no dependencies). I don't know
if we need to preserve this too, but the tests fail if we don't.

It was super fun to see one test assert it was optional and fail, only
to have the test that asserts it as required then fail (:

src/Database/Schema/Migrations/Filesystem.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE DeriveDataTypeable, DeriveGeneric, ScopedTypeVariables, OverloadedStrings #-}
+{-# LANGUAGE DeriveDataTypeable, DeriveGeneric, LambdaCase, ScopedTypeVariables, OverloadedStrings #-}
 -- |This module provides a type for interacting with a
 -- filesystem-backed 'MigrationStore'.
 module Database.Schema.Migrations.Filesystem
@@ -28,6 +28,7 @@ import Control.Monad ( filterM )
 import Control.Exception ( Exception(..), throw, catch )
 
 import Data.Aeson
+import Data.Aeson.Types (typeMismatch)
 import Data.HashMap.Strict as M (toList)
 import qualified Data.Yaml as Yaml
 import GHC.Generics (Generic)
@@ -113,8 +114,7 @@ data MigrationYaml = MigrationYaml
     , myDescription :: Maybe Text
     , myApply :: Text
     , myRevert :: Maybe Text
-    , myDepends :: Maybe Text
-    -- ^ White-space separated names
+    , myDepends :: DependsYaml
     }
     deriving Generic
 
@@ -139,7 +139,7 @@ migrationYamlToMigration theId my = Migration
     , mDesc = myDescription my
     , mApply = myApply my
     , mRevert = myRevert my
-    , mDeps = maybe [] T.words $ myDepends my
+    , mDeps = unDependsYaml $ myDepends my
     }
 
 newtype UTCTimeYaml = UTCTimeYaml
@@ -159,3 +159,21 @@ instance ToJSON UTCTimeYaml where
 -- Keeps things as the old Show/Read-based format, e.g "2009-04-15 10:02:06 UTC"
 utcTimeYamlFormat :: String
 utcTimeYamlFormat = "%F %T UTC"
+
+newtype DependsYaml = DependsYaml
+  { unDependsYaml :: [Text]
+  }
+
+instance FromJSON DependsYaml where
+    parseJSON = \case
+        Null -> pure $ DependsYaml []
+        String t -> pure $ DependsYaml $ T.words t
+        x -> typeMismatch "Null or whitespace-separated String" x
+
+instance ToJSON DependsYaml where
+    toJSON (DependsYaml ts) = case ts of
+        [] -> toJSON Null
+        _ -> toJSON $ T.unwords ts
+    toEncoding (DependsYaml ts) = case ts of
+        [] -> toEncoding Null
+        _ -> toEncoding $ T.unwords ts