libstore: check additionalSandboxProfile

Make sure that `extraSandboxProfile` is set before we check whether it's
empty or not (in the `sandbox=true` case).

Also adds a test case for this.

Co-Authored-By: Artemis Tosini <lix@artem.ist>
Co-Authored-By: Eelco Dolstra <edolstra@gmail.com>

tests/functional/extra-sandbox-profile.nix

@@ -0,0 +1,19 @@
+{ destFile, seed }:
+
+with import ./config.nix;
+
+mkDerivation {
+  name = "simple";
+  __sandboxProfile = ''
+    # Allow writing any file in the filesystem
+    (allow file*)
+  '';
+  inherit seed;
+  buildCommand = ''
+    (
+      set -x
+      touch ${destFile}
+      touch $out
+    )
+  '';
+}