Fix Geo checksummable check failing when file is nil

In specific cases, such as the LFS download flow,
we would first create a model before updating it to contain
a file as well, which means it's possible the model's file
doesn't exist.

We have an after_commit hook that attempts to verify if
a record's file is checksummable to run verification on,
however this would have failed if the file was nil before this.

Changelog: fixed
EE: true

ee/app/models/concerns/geo/blob_replicator_strategy.rb

@@ -95,7 +95,7 @@ module Geo
     #
     # @return [Boolean] whether the file exists on disk or in remote storage
     def file_exists?
-      carrierwave_uploader.file.exists?
+      carrierwave_uploader.file&.exists?
     end
 
     def deleted_params

ee/spec/support/shared_examples/models/concerns/blob_replicator_strategy_shared_examples.rb

@@ -231,4 +231,29 @@ RSpec.shared_examples 'a blob replicator' do
       end
     end
   end
+
+  describe '#file_exists?' do
+    let(:file) { double(exists?: true) }
+    let(:uploader) { double(file: file) }
+
+    subject { replicator.file_exists? }
+
+    before do
+      allow(replicator).to receive(:carrierwave_uploader).and_return(uploader)
+    end
+
+    it { is_expected.to be_truthy }
+
+    context 'when the file does not exist' do
+      let(:file) { double(exists?: false) }
+
+      it { is_expected.to be_falsey }
+    end
+
+    context 'when the file is nil' do
+      let(:file) { nil }
+
+      it { is_expected.to be_falsey }
+    end
+  end
 end