fix #39233 - 500 in merge request

- handle unchanged empty lines in inline diff

changelogs/unreleased/15774-fix-39233-500-in-merge-request.yml

+---
+title: 'fix #39233 - 500 in merge request'
+merge_request: 15774
+author: Martin Nowak
+type: fixed

lib/gitlab/diff/inline_diff.rb

@@ -70,7 +70,7 @@ def for_lines(lines)
         def find_changed_line_pairs(lines)
           # Prefixes of all diff lines, indicating their types
           # For example: `" - +  -+  ---+++ --+  -++"`
-          line_prefixes = lines.each_with_object("") { |line, s| s << line[0] }.gsub(/[^ +-]/, ' ')
+          line_prefixes = lines.each_with_object("") { |line, s| s << (line[0] || ' ') }.gsub(/[^ +-]/, ' ')
 
           changed_line_pairs = []
           line_prefixes.scan(LINE_PAIRS_PATTERN) do

spec/lib/gitlab/diff/inline_diff_spec.rb

@@ -31,6 +31,10 @@ class Test
       expect(subject[7]).to eq([17..17])
       expect(subject[8]).to be_nil
     end
+
+    it 'can handle unchanged empty lines' do
+      expect { described_class.for_lines(['- bar', '+ baz', '']) }.not_to raise_error
+    end
   end
 
   describe "#inline_diffs" do