Fix blender when line is None

[sbrunner]

Description

Currently, we can obtain this exceptions:

Traceback (most recent call last):
  File "/home/runner/.cache/pre-commit/repocjulckrl/py_env-python3/bin/prospector", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/runner/.cache/pre-commit/repocjulckrl/py_env-python3/lib/python3.11/site-packages/prospector/run.py", line 200, in main
    prospector.execute()
  File "/home/runner/.cache/pre-commit/repocjulckrl/py_env-python3/lib/python3.11/site-packages/prospector/run.py", line 119, in execute
    messages = self.process_messages(found_files, messages)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/.cache/pre-commit/repocjulckrl/py_env-python3/lib/python3.11/site-packages/prospector/run.py", line 30, in process_messages
    messages = blender.blend(messages)
               ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/.cache/pre-commit/repocjulckrl/py_env-python3/lib/python3.11/site-packages/prospector/blender.py", line 91, in blend
    assert message.location.line is not None
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Then fix it

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[Pierre-Sassoulas]

LGTM !

[Pierre-Sassoulas]

Created 1.13.2 and 1.14.0 milestone, I don't know if you want to keep having those, but imo fixing bugs fast and releasing is what's important (changelog and properly labelled issue on github are nice to have).