runProgram(): Distinguish between empty input and no input

For example, if we call brotli with an empty input, it shouldn't read
from the caller's stdin.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index da29b2fc6fc6..22bde086e6a2 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -671,7 +671,7 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
             Path tmpDir = createTempDir();
             AutoDelete autoDelete(tmpDir, true);
             // FIXME: this requires GNU tar for decompression.
-            runProgram("tar", true, {"xf", storePath, "-C", tmpDir, "--strip-components", "1"}, "");
+            runProgram("tar", true, {"xf", storePath, "-C", tmpDir, "--strip-components", "1"});
             unpackedStorePath = store->addToStore(name, tmpDir, true, htSHA256, defaultPathFilter, false);
         }
         replaceSymlink(unpackedStorePath, unpackedLink);