gn, mac: Work without a sysroot

This is needed to make the “sysroot = "/"” configuration, which
translates to “sysroot = ""”, work properly

Bug: crashpad:79
Change-Id: I25ab49b7d57abfcf0ce9a62925013bb58dadf5dd
Reviewed-on: https://chromium-review.googlesource.com/831007
Reviewed-by: Scott Graham <scottmg@chromium.org>

util/BUILD.gn

@@ -42,9 +42,13 @@ if (is_mac) {
         hermetic_xcode_path,
       ]
     }
+    if (sysroot != "") {
+      args += [
+        "--sdk",
+        sysroot,
+      ]
+    }
     args += [
-      "--sdk",
-      sysroot,
       "--include",
       rebase_path("../compat/mac", root_build_dir),
     ]