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.sh

@@ -0,0 +1,23 @@
+source common.sh
+
+if [[ $(uname) != Darwin ]]; then skipTest "Need Darwin"; fi
+
+DEST_FILE="${TEST_ROOT}/foo"
+
+testSandboxProfile () (
+    set -e
+
+    sandboxMode="$1"
+
+    rm -f "${DEST_FILE}"
+    nix-build --no-out-link ./extra-sandbox-profile.nix \
+        --option sandbox "$sandboxMode" \
+        --argstr seed "$RANDOM" \
+        --argstr destFile "${DEST_FILE}"
+
+    ls -l "${DEST_FILE}"
+)
+
+testSandboxProfile "false"
+expectStderr 2 testSandboxProfile "true"
+testSandboxProfile "relaxed"