Passing release manifest to build script without branding causes error

[mthalman]

See #5335 (comment)

$ ./build.sh --source-only --release-manifest release.json
ERROR: Failed to find branding in /release.json

We don't provide any branding field in the release manifest, which is expected as that won't be happening for 10.0 anymore. But the build script is expecting it to be there.