fix: Treat empty TMPDIR as unset

Fixes an instance of

    nix: src/libutil/util.cc:139: nix::Path nix::canonPath(PathView, bool): Assertion `path != ""' failed.

... which I've been getting in one of my shells for some reason.
I have yet to find out why TMPDIR was empty, but it's no reason for
Nix to break.

(cherry picked from commit c3fb2aa1f9)

src/libutil/file-system.hh

@@ -0,0 +1,17 @@
+#pragma once
+/**
+ * @file
+ *
+ * Utiltities for working with the file sytem and file paths.
+ */
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Return `TMPDIR`, or the default temporary directory if unset or empty.
+ */
+Path defaultTempDir();
+
+}