Fix display of some overflowing merge request diffs

MR diffs that overflow may create blobs where `ruby_encoding: nil`.

changelogs/unreleased/199843-fix-viewing-nil-blobs.yml

+---
+title: Fix display of some overflowing merge request diffs
+merge_request: 29267
+author:
+type: fixed

lib/gitlab/blob_helper.rb

@@ -3,6 +3,8 @@
 # This has been extracted from https://github.com/github/linguist/blob/master/lib/linguist/blob_helper.rb
 module Gitlab
   module BlobHelper
+    include Gitlab::Utils::StrongMemoize
+
     def extname
       File.extname(name.to_s)
     end
@@ -120,8 +122,18 @@ module Gitlab
     end
 
     def encoded_newlines_re
-      @encoded_newlines_re ||=
-        Regexp.union(["\r\n", "\r", "\n"].map { |nl| nl.encode(ruby_encoding, "ASCII-8BIT").force_encoding(data.encoding) })
+      strong_memoize(:encoded_newlines_re) do
+        newlines = ["\r\n", "\r", "\n"]
+        data_encoding = data&.encoding
+
+        if ruby_encoding && data_encoding
+          newlines.map! do |nl|
+            nl.encode(ruby_encoding, "ASCII-8BIT").force_encoding(data_encoding)
+          end
+        end
+
+        Regexp.union(newlines)
+      end
     end
 
     def ruby_encoding

spec/lib/gitlab/git/blob_spec.rb

@@ -652,4 +652,16 @@ describe Gitlab::Git::Blob, :seed_helper do
       expect(described_class).to respond_to(:gitlab_blob_size)
     end
   end
+
+  describe '#lines' do
+    context 'when the encoding cannot be detected' do
+      it 'successfully splits the data' do
+        data = "test\nblob"
+        blob = Gitlab::Git::Blob.new(name: 'test', size: data.bytesize, data: data)
+        expect(blob).to receive(:ruby_encoding) { nil }
+
+        expect(blob.lines).to eq(data.split("\n"))
+      end
+    end
+  end
 end

spec/support/helpers/fake_blob_helpers.rb

@@ -22,7 +22,11 @@ module FakeBlobHelpers
     alias_method :name, :path
 
     def id
-      0
+      "00000000"
+    end
+
+    def commit_id
+      "11111111"
     end
 
     def binary_in_repo?