remote: fix queryPathInfoUncached missing content address decoding

queryPathInfoUncached fails with a "not enough input" message when
decoding Metadata that is missing a content address.

nix-daemon encodes a missing content address as an empty string:
https://github.com/NixOS/nix/blob/a6b315ae8/src/libstore/content-address.cc#L139

Change the decoding to return Nothing if the caString is empty.

Author: squalus

hnix-store-remote/src/System/Nix/Store/Remote.hs

@@ -34,6 +34,7 @@ import Data.Dependent.Sum (DSum((:=>)))
 import Data.HashSet (HashSet)
 import Data.Map (Map)
 import Data.Text (Text)
+import qualified Data.Text
 import qualified Control.Monad
 import qualified Data.Attoparsec.Text
 import qualified Data.Text.Encoding
@@ -253,6 +254,7 @@ queryPathInfoUncached path = do
       sigs = Data.Set.empty
 
       contentAddress =
+        if Data.Text.null caString then Nothing else
         case
           Data.Attoparsec.Text.parseOnly
             System.Nix.ContentAddress.contentAddressParser