v4.0.2-rc3

Add placeholder google-protobuf.js file to fix missing input

Release workflow currently errors out with `//:dist_files: missing
input file '//:google-protobuf.js'` because we normally only generate
it as part of the cross-compiled binary and npm releases. However,
this seems to be causing problems for the BCR release process because
the build target is broken without it.

This change just adds a placeholder file to smooth things over.