fix: Handle empty lines in patch parser

[neubig]

This PR fixes an issue where the patch parser was not correctly handling empty lines in patches, causing patches to fail when trying to modify lines after an empty line.

The fix involves:

1. Treating empty lines in the patch as context lines with empty content
2. Adding a test case to verify the fix

Fixes #6140

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:479a7c7-nikolaik   --name openhands-app-479a7c7   docker.all-hands.dev/all-hands-ai/openhands:479a7c7

[enyst]

Thank you! It's worth trying this live ASAP, IMHO, since it's broken out there and this changes only the patch processing.