client: remove empty blob handling
client: remove empty blob handling

This is not necessary now per
https://github.com/bazelbuild/remote-apis/blob/6345202a036a297b22b0a0e7531ef702d05f2130/build/bazel/remote/execution/v2/remote_execution.proto#L249

Bug: 1143123
Change-Id: If2fef91d7d1766a5e2658edf5127e14e5b2daad6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2739096
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py