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

tools/releasetools/fsverity_metadata_generator.py

@@ -178,6 +178,7 @@ class FSVerityMetadataGenerator:
           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: