RFR: Fix webrev generation when target hunk is empty

[Erik Duveblad via github.com]

Hi all,

this patch similar to one Robin did a few days ago. Robin fixed generation of webrevs when a source hunk in a patch was empty, this change fixes generation of webrevs when a target hunk in the patch is empty and starts at 0 (i.e. the user has removed lines from the start of a file).

## Testing
- [x] `sh gradlew test` passes on Linux x86_64
- Added new unit test

Thanks,
Erik

----------------

Commits:
 - db9b79c6:	Fix webrev generation when target hunk is empty

Pull request:
https://git.openjdk.java.net/skara/pull/82

Webrev:
https://webrevs.openjdk.java.net/skara/82/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/82.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/82/head:pull/82