DifferenceEngine: Don't display empty header row

This prevents a gap from appearing, and the resulting code will be
slightly cleaner as well.

Shouldn't matter in core, but does matter for extensions displaying
diffs like AbuseFilter.

Change-Id: I9e3e74226a23c0a18db091bc1550b694d9d08118
Merge "DifferenceEngine: Don't display empty header row"