[SuperSize] Fix breakage on Trichrome from |metadata| being None.

Previously, CreateMetadata() returns None if the resulting metadata is
empty. Recently updates to _ParseApkOtherSymbols() requires |metadata|
to be available (to add 'zipalign_padding') field. This leads to
breakage when SuperSize is run on Trichrome.

This CL changes CreateMetadata() to return {} instead of None to fix
the issue. This does not affect existing metadata use in SuperSize;
already it's not explicitly compared against None.

Bug: 1040645
Change-Id: I8664148f647910cbb5f9dea216684262174e0e03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427324Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810255}

tools/binary_size/libsupersize/archive.py

@@ -852,7 +852,7 @@ def CreateMetadata(map_path, elf_path, apk_path, minimal_apks_path, tool_prefix,
         key += '-' + name
       metadata[key] = size
 
-  return metadata or None
+  return metadata
 
 
 def _ResolveThinArchivePaths(raw_symbols, thin_archives):