only set default values for nix-path if nothing else is set

TODO: this doesn't work yet.
we currently have no way of checking if a config value was set or not.

this is the expected behavior for default values:
new values should override them, not prepend.

the change fixes a logic bug introduced when fixing the previously confused
override mechanism in e062021314.

src/libexpr/eval-gc.cc

@@ -101,7 +101,7 @@ void initGC()
 #endif
 
     // NIX_PATH must override the regular setting
-    // See the comment in applyConfig
+    // See the XXX comment in `applyConfig()`
     if (auto nixPathEnv = getEnv("NIX_PATH")) {
         globalConfig.set("nix-path", concatStringsSep(" ", EvalSettings::parseNixPath(nixPathEnv.value())));
     }

src/libexpr/eval.cc

@@ -341,6 +341,7 @@ EvalState::EvalState(
         for (auto & i : settings.nixPath.get()) {
             lookupPath.elements.emplace_back(LookupPath::Elem::parse(i));
         }
+        // TODO: only set if nothing else is
         if (!settings.restrictEval) {
             for (auto & i : EvalSettings::getDefaultNixPath()) {
                 lookupPath.elements.emplace_back(LookupPath::Elem::parse(i));

tests/functional/nix_path.sh

@@ -42,6 +42,12 @@ done
 # finding something that's not in any of the default paths fails
 ( ! $(nix-instantiate --find-file test) )
 
+# setting anything overrides the default paths
+# this ensures we can force an empty search path
+[[ $(NIX_PATH= nix-instantiate --eval -E 'with builtins; length nixPath') = 0 ]]
+[[ $(nix-instantiate --nix-path "" --eval -E 'with builtins; length nixPath') = 0 ]]
+[[ $(nix-instantiate -I "" --eval -E 'with builtins; length nixPath') = 1 ]]
+
 echo "nix-path = test=$TEST_ROOT/from-nix-path-file" >> "$test_nix_conf"
 
 # Use nix.conf in absence of NIX_PATH