Fix handling of optional fields in coerce

[psyho]

A field that appears in a Hash as nil breaks coercing the Hash with an error like: "Value must be a Hash or a Foo (ArgumentError)".

For example, the version 0.19.0 of gapic-common breaks google-cloud-firestore.

Running this:

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'dotenv'
  gem 'gapic-common', '0.19.0'
  gem 'google-cloud-firestore', '2.13.0'
end

require 'dotenv/load'
require 'google/cloud/firestore'

firestore = Google::Cloud::Firestore.new(project_id: ENV.fetch('PROJECT_ID'))
doc = firestore.doc(ENV.fetch('DOCUMENT_ID'))
puts doc.get

Results in an error like:

/home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/gapic-common-0.19.0/lib/gapic/protobuf.rb:39:in `coerce': Value  must be a Hash or a Google::Cloud::Firestore::V1::DocumentMask (ArgumentError)
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/gapic-common-0.19.0/lib/gapic/protobuf.rb:99:in `coerce_submessage'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/gapic-common-0.19.0/lib/gapic/protobuf.rb:61:in `block in coerce_submessages'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/gapic-common-0.19.0/lib/gapic/protobuf.rb:57:in `each'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/gapic-common-0.19.0/lib/gapic/protobuf.rb:57:in `coerce_submessages'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/gapic-common-0.19.0/lib/gapic/protobuf.rb:41:in `coerce'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/google-cloud-firestore-v1-0.10.0/lib/google/cloud/firestore/v1/firestore/client.rb:749:in `batch_get_documents'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/google-cloud-firestore-2.13.0/lib/google/cloud/firestore/service.rb:74:in `get_documents'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/google-cloud-firestore-2.13.0/lib/google/cloud/firestore/client.rb:348:in `get_all'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/google-cloud-firestore-2.13.0/lib/google/cloud/firestore/document_reference.rb:168:in `each'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/google-cloud-firestore-2.13.0/lib/google/cloud/firestore/document_reference.rb:168:in `first'
        from /home/psyho/.rbenv/versions/3.1.3/lib/ruby/gems/3.1.0/gems/google-cloud-firestore-2.13.0/lib/google/cloud/firestore/document_reference.rb:168:in `get'
        from ./gapic:18:in `<main>'

[dazuma]

Thanks!