Write signature size even when it's empty

Normally this has no effect, but when we generate metadata for small
files (<=4KB), merkle tree isn't generated. In such case, writing zero
will make the metadata format simpler and unconditional.

Test: manual
Change-Id: Ibe18175b580af3409c896a8bb97323792ad9c459
diff --git a/tools/releasetools/fsverity_metadata_generator.py b/tools/releasetools/fsverity_metadata_generator.py
index a300d2e..fa7cd39 100644
--- a/tools/releasetools/fsverity_metadata_generator.py
+++ b/tools/releasetools/fsverity_metadata_generator.py
@@ -178,6 +178,7 @@
           out.write(sig)
       else:
         out.write(pack('<I', SIG_TYPE_NONE))
+        out.write(pack('<I', 0))
 
       # 4. merkle tree
       with open(merkletree_file, 'rb') as f: