Fix display of some overflowing merge request diffs

What does this MR do?

MR diffs that overflow may create blobs where ruby_encoding: nil. We fail to handle that properly in BlobHelper#encoded_newlines_re.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #199843 (closed)