create_brick_ota: fix nonetype bug

If devkeys are present, we don't need to specify a serialno. In this
case we should conditionally add serial no to our metadata

Test: create_brick_ota
Change-Id: I9c93aa0f9ec5752693a9ddde5b76be4c12d6fc3b
diff --git a/tools/releasetools/create_brick_ota.py b/tools/releasetools/create_brick_ota.py
index f290323..9e040a5 100644
--- a/tools/releasetools/create_brick_ota.py
+++ b/tools/releasetools/create_brick_ota.py
@@ -45,13 +45,17 @@
   partitions_to_wipe = PARTITIONS_TO_WIPE
   if extra_wipe_partitions is not None:
     partitions_to_wipe = PARTITIONS_TO_WIPE + extra_wipe_partitions.split(",")
+    ota_metadata = ["ota-type=BRICK", "post-timestamp=9999999999",
+                    "pre-device=" + product_name]
+    if serialno is not None:
+        ota_metadata.append("serialno=" + serialno)
   # recovery requiers product name to be a | separated list
   product_name = product_name.replace(",", "|")
   with zipfile.ZipFile(output_path, "w") as zfp:
     zfp.writestr("recovery.wipe", "\n".join(partitions_to_wipe))
     zfp.writestr("payload.bin", "")
     zfp.writestr("META-INF/com/android/metadata", "\n".join(
-        ["ota-type=BRICK", "post-timestamp=9999999999", "pre-device=" + product_name, "serialno=" + serialno]))
+        ota_metadata))
 
 
 def main(argv):