async job finder: correctly handle files not ending in newline

[emmanueltouzery]

fixes #2517

Description

The telescope async job runner would not properly handle files without a trailing newline. It would just crash on them.

Fixes #2517

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list relevant details about your configuration

• the ast-grep issue as described in Encountered an error when using git_command in git_bcommits #2517
• a simple telescope live_grep query to confirm the behavior with trailing newlines is still OK

Configuration:

• Neovim version 0.11.1
• Operating system and version: linux fedora 42

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)