direnv: simplify, work around nushell/nushell#14112

nushell 0.99 does not like early returns in hooks. So, what if we just
didn't? Rewrite the entire hook to work as one single pipeline.

modules/programs/direnv.nix

@@ -162,28 +162,20 @@ in {
           $env.config.hooks.pre_prompt?
           | default []
           | append {||
-              let direnv = (
               ${direnvWrapped}
               | from json --strict
               | default {}
-              )
-              if ($direnv | is-empty) {
-                  return
-              }
-              $direnv
               | items {|key, value|
-                  {
-                      key: $key
-                      value: (do (
+                  let value = do (
                       $env.ENV_CONVERSIONS?
                       | default {}
                       | get -i $key
                       | get -i from_string
                       | default {|x| $x}
-                      ) $value)
+                  ) $value
+                  return [ $key $value ]
               }
-              }
-              | transpose -ird
+              | into record
               | load-env
           }
       )