Accept empty string as empty json body when calling an rpc (#832)

src/PostgREST/ApiRequest.hs

@@ -13,6 +13,7 @@ module PostgREST.ApiRequest ( ApiRequest(..)
 
 import           Protolude
 import qualified Data.Aeson                as JSON
+import           Data.Aeson.Types          (emptyObject)
 import qualified Data.ByteString           as BS
 import qualified Data.ByteString.Internal  as BS (c2w)
 import qualified Data.ByteString.Lazy      as BL
@@ -124,13 +125,13 @@ userApiRequest schema req reqBody
   payload =
     case decodeContentType . fromMaybe "application/json" $ lookupHeader "content-type" of
       CTApplicationJSON ->
-          either Left (\val -> case ensureUniform (pluralize val) of
-            Nothing -> Left "All object keys must match"
-            Just json -> Right json) (JSON.eitherDecode reqBody)
+        note "All object keys must match" . ensureUniform . pluralize
+          =<< if BL.null reqBody && isTargetingProc
+               then Right emptyObject
+               else JSON.eitherDecode reqBody
       CTTextCSV ->
-          either Left (\val -> case ensureUniform (csvToJson val) of
-            Nothing -> Left "All lines must have same number of fields"
-            Just json -> Right json) (CSV.decodeByName reqBody)
+        note "All lines must have same number of fields" . ensureUniform . csvToJson
+          =<< CSV.decodeByName reqBody
       CTOther "application/x-www-form-urlencoded" ->
         Right . PayloadJSON . V.singleton . M.fromList
                     . map (toS *** JSON.String . toS) . parseSimpleQuery