[Fix] integrate_inputlinks if no published instance where found

[jlorrain]

integrate_inputlinks throw an error if nothing was published

Traceback (most recent call last):
  File "C:\Users\jerome.lorrain\PycharmProjects\OpenPype_build\.venv\lib\site-packages\pyblish\plugin.py", line 522, in __explicit_process
    runner(*args)
  File "C:\Users\jerome.lorrain\PycharmProjects\OpenPype_build\openpype\plugins\publish\integrate_inputlinks.py", line 85, in process
  File "C:\Users\jerome.lorrain\PycharmProjects\OpenPype_build\openpype\plugins\publish\integrate_inputlinks.py", line 121, in write_links_to_database
AttributeError: 'NoneType' object has no attribute 'data'

Fix to not throw an error (but still log a warning)

[iLLiCiTiT]

Already fixed with #2322