Fix diff file creation

During import of huge project mentoined in 37682, we encountered two different errors that were breaking the import.

They are all related to creating DiffFile:

undefined method `index' for nil:NilClass

and

undefined method `line_for_position' for nil:NilClass"

This MR is trying to prevent the import from breaking, and fixing those errors.

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

#37682 (closed)