Fix REPL autocomplete (#837)

This commit fixes an issue that made the last branch of the completer
unreachable.

The second to last branch was always `True`:

```
> split (== '.') "anything"
["anything"]
```

Signed-off-by: Basile Henry <bjm.henry@gmail.com>

dhall/src/Dhall/Repl.hs

@@ -455,6 +455,7 @@ completeFunc reversedPrev word
 
   -- Complete record fields and union alternatives
   | var : subFields <- Text.split (== '.') (Text.pack word)
+  , not $ null subFields
   = do
     Env { envBindings } <- get