Fix encoding detection for nil diffs

app/models/merge_request_diff.rb

@@ -509,6 +509,8 @@ class MergeRequestDiff < ApplicationRecord
   end
 
   def encode_in_base64?(diff_text)
+    return false if diff_text.nil?
+
     (diff_text.encoding == Encoding::BINARY && !diff_text.ascii_only?) ||
       diff_text.include?("\0")
   end
@@ -536,7 +538,7 @@ class MergeRequestDiff < ApplicationRecord
       rows.each do |row|
         data = row.delete(:diff)
         row[:external_diff_offset] = file.pos
-        row[:external_diff_size] = data.bytesize
+        row[:external_diff_size] = data&.bytesize || 0
 
         file.write(data)
       end

changelogs/unreleased/fix-encoding-for-nil-diff-files.yml

+---
+title: Fix migrating some empty diffs
+merge_request: 42825
+author:
+type: fixed

spec/models/merge_request_diff_spec.rb

@@ -180,6 +180,17 @@ RSpec.describe MergeRequestDiff do
       expect(diff.external_diff_store).to eq(file_store)
     end
 
+    it 'migrates a nil diff file' do
+      expect(diff).not_to be_stored_externally
+      MergeRequestDiffFile.where(merge_request_diff_id: diff.id).update_all(diff: nil)
+
+      stub_external_diffs_setting(enabled: true)
+
+      diff.migrate_files_to_external_storage!
+
+      expect(diff).to be_stored_externally
+    end
+
     it 'safely handles a transaction error when migrating to external storage' do
       expect(diff).not_to be_stored_externally
       expect(diff.external_diff).not_to be_exists