Stop resolving locally for empty CHECKSUMS entries

[hsbt]

Fixes #9109.

Bundler.setup without a lockfile writes CHECKSUMS entries without digests, because nothing is fetched on that path. Since 05199ae those empty entries count as a lockfile change, so every later Bundler.setup printed "Resolving dependencies..." and re-resolved although no local resolution can ever fill them. The lockfile ends up identical each time.

I now count an empty entry only when a remote fetch is coming or when frozen mode has to refuse it. bundle install still goes remote to fill those entries, and frozen mode still raises the same error. As a side effect bundle install --local stops re-resolving for them too, which is the path the original report hit through Fedora's package build.

I did not compute the digests from installed gems on the setup path. Default gems have no .gem file to hash, and distributions like Fedora strip the gem cache, so the entries would stay empty there and the loop would remain.

$ printf 'source "https://rubygems.org"\ngem "minitest"\n' > Gemfile
$ printf 'require "bundler"\nBundler.setup\n' > setup.rb
$ ruby setup.rb   # resolves and writes the lockfile
Resolving dependencies...
$ ruby setup.rb   # before: resolved again, after: silent, lockfile untouched

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