Ruby: Issue decoding wrapped values after upgrading from 3.10.1 to 3.11.1

[makushline]

$ cat repo.rb
require 'google/protobuf'
require 'google/protobuf/wrappers_pb'
Google::Protobuf::DescriptorPool.generated_pool.build do
  add_message "Person" do
    optional :name, :message, 1, "google.protobuf.StringValue"
  end
end
Person  = Google::Protobuf::DescriptorPool.generated_pool.lookup("Person").msgclass
puts "before decoding"
puts hi = Person.new(:name => {:value => "hi"})
puts empty = Person.new(:name => {:value => ""})
puts not_set = Person.new()
puts "after decoding"
puts Person.decode(hi.to_proto)
puts Person.decode(empty.to_proto)
puts Person.decode(not_set.to_proto)

// at google-protobuf 3.11.0
$ ruby repo.rb 
before decoding
<Person: name: <Google::Protobuf::StringValue: value: "hi">>
<Person: name: <Google::Protobuf::StringValue: value: "">>
<Person: name: nil>
after decoding
<Person: name: <Google::Protobuf::StringValue: value: "hi">>
<Person: name: nil>
<Person: name: nil>

// at google-protobuf 3.10.1
$ ruby repo.rb
before decoding
<Person: name: <Google::Protobuf::StringValue: value: "hi">>
<Person: name: <Google::Protobuf::StringValue: value: "">>
<Person: name: nil>
after decoding
<Person: name: <Google::Protobuf::StringValue: value: "hi">>
<Person: name: <Google::Protobuf::StringValue: value: "">>
<Person: name: nil>

$ cat Gemfile.lock
source "https://rubygems.org"
gem "google-protobuf", "3.10.1"

I expect the decoded Person with an empty name to look like
<Person: name: <Google::Protobuf::StringValue: value: "">>

[haberman]

Could you try this PR and verify that it fixes your problem?

#7195