direnv: Apply nushell env transformations

In nushell, the ENV_CONVERSIONS environment variable is used to
transform the defined variables from a string to a nushell value (PATH
to a list being one of the most common uses). This commit applies
user-defined conversions to direnv-loaded variables.

This fixes binary autocompletion not being triggered for newly added
paths and makes direnv work consistently with nushell

tests/modules/programs/direnv/nushell.nix

@@ -14,6 +14,6 @@
   in ''
     assertFileExists "${configFile}"
     assertFileRegex "${configFile}" \
-      'let direnv = (/nix/store/.*direnv.*/bin/direnv export json | from json)'
+      'let direnv = (/nix/store/.*direnv.*/bin/direnv export json \| from json | default {})'
   '';
 }