fix #2873 WM_NAME now shows "not accessible" in non-existent directories

[80KiloMett]

ISSUE TYPE

• Bug fix

RUNTIME ENVIRONMENT

• Operating system and version: Arch Linux
• Terminal emulator and version: Alacritty 0.12.2
• Python version: 3.11.5
• Ranger version/commit: 136416c
• Locale: en_US.UTF-8

CHECKLIST

• The CONTRIBUTING document has been read [REQUIRED]
• All changes follow the code style [REQUIRED]
• All new and existing tests pass [REQUIRED]
  • I did not run spellcheck HACKING was not clear which tool to use specifically
• Changes require config files to be updated
  • Config files have been updated
• Changes require documentation to be updated
  • Documentation has been updated
• Changes require tests to be updated
  • Tests have been updated

DESCRIPTION

fixes #2873
Before ranger used to crash with update_title=true setting when trying to display a non-existent directory.
The reason was a call to UI.fm.thisdir.path while UI.fm.thisdir == None when ranger is pointing to a non-existent directory, invalidating the path attribute.
This PR checks if UI.fm.thisdir is None and makes sure WM_NAME shows ranger:not accessible (same as the ranger UI).

MOTIVATION AND CONTEXT

fix crashing issue detailed in #2873

TESTING

make test_py has been run successfully. I was not sure which spellchecker to get, HACKING wasn't super clear on this, sorry.

Changes should not have any effects on the rest of the codebase.