Fix sync PRs failing when the pipeline is missing manifest.author

[mirpedrol]

Description of the bug

Example: https://github.com/nf-core/tools/actions/runs/12431872066/job/34714209849

Command used and terminal output

,--./,-.
          ___     __   __   __   ___     /,-._.--~\ 
    |\ | |__  __ /  ` /  \ |__) |__         }  {
    | \| |       \__, \__/ |  \ |___     \`-._,-`-,
                                          `._,._,'

    nf-core/tools version 3.1.1 - https://nf-co.re/


WARNING  Could not find GitHub authentication token. Some API requests may fail.
WARNING  Could not find GitHub authentication token. Some API requests may fail.
INFO     Pipeline directory:                                                    
         /opt/actions-runner/_work/tools/tools/nf-core/multiplesequencealign    
INFO     Using branch 'dev' to fetch workflow variables                         
INFO     Will attempt to automatically create a pull request                    
INFO     Original pipeline repository branch is 'dev'                           
ERROR    Workflow config variable `manifest.author` not found!                  
Error: Process completed with exit code 1.

System information

No response

[mirpedrol]

Done in #3397