Update TarReader::Entry#read expectations for ruby 3.2

In ruby 3.1 and earlier, reading from an empty tar entry would return
nil.  Now this is not the case, so update our expectations.

We run this in the pipeline on a docker image with ruby 3.2, so also
update the .ruby-version file in this repo.

ref: ruby/ruby@65ca14e

Author: selzoc

.ruby-version

@@ -1 +1 @@
-3.1
+3.2

spec/integration/build/aws_spec.rb

@@ -100,7 +100,7 @@
       apply_spec = JSON.parse(read_from_tgz(stemcell, 'apply_spec.yml'))
       expect(apply_spec['agent_commit']).to eq(@agent_commit)
 
-      expect(read_from_tgz(stemcell, 'image')).to be_nil
+      expect(read_from_tgz(stemcell, 'image')).to be_empty
       expect(File.read(stemcell_sha)).to eq(Digest::SHA1.hexdigest(File.read(stemcell)))
 
       # running task should create packer-output-ami.txt in AMIS_DIR
@@ -277,7 +277,7 @@
       apply_spec = JSON.parse(read_from_tgz(stemcell, 'apply_spec.yml'))
       expect(apply_spec['agent_commit']).to eq(@agent_commit)
 
-      expect(read_from_tgz(stemcell, 'image')).to be_nil
+      expect(read_from_tgz(stemcell, 'image')).to be_empty
       expect(File.read(stemcell_sha)).to eq(Digest::SHA1.hexdigest(File.read(stemcell)))
     end
   end

spec/integration/build/azure_spec.rb

@@ -100,7 +100,7 @@
 
       # expect(read_from_tgz(stemcell, 'updates.txt')).to eq('some-updates')
 
-      expect(read_from_tgz(stemcell, 'image')).to be_nil
+      expect(read_from_tgz(stemcell, 'image')).to be_empty
       expect(File.read(stemcell_sha)).to eq(Digest::SHA1.hexdigest(File.read(stemcell)))
     end
   end

spec/integration/build/gcp_spec.rb

@@ -73,7 +73,7 @@
       apply_spec = JSON.parse(read_from_tgz(stemcell, 'apply_spec.yml'))
       expect(apply_spec['agent_commit']).to eq(agent_commit)
 
-      expect(read_from_tgz(stemcell, 'image')).to be_nil
+      expect(read_from_tgz(stemcell, 'image')).to be_empty
     end
   end
 end