test: add test for non-defaulting for stding installable input

src/libcmd/installables.cc

@@ -701,7 +701,7 @@ RawInstallablesCommand::RawInstallablesCommand()
 {
     addFlag({
         .longName = "stdin",
-        .description = "Read installables from the standard input.",
+        .description = "Read installables from the standard input. No default installable applied.",
         .handler = {&readFromStdIn, true}
     });
 

tests/build.sh

@@ -129,3 +129,7 @@ nix build --impure -f multiple-outputs.nix --json e --no-link | jq --exit-status
     (.drvPath | match(".*multiple-outputs-e.drv")) and
     (.outputs | keys == ["a_a", "b"]))
 '
+
+# Make sure that `--stdin` works and does not apply any defaults
+printf "" | nix build --no-link --stdin --json | jq --exit-status '. == []'
+printf "%s\n" "$drv^*" | nix build --no-link --stdin --json | jq --exit-status '.[0]|has("drvPath")'