Rename image script fails when calling `build_substitute_list`

[fabiangebert]

I am trying to rename using the image renamer. It used to work perfectly well approx 2-3 months ago. May be related to Darktable update?
Running Darktable 5.4.1 on macOS 26.4.

    25.6720 LUA ERROR : [HOME]/.config/darktable/lua/lib/dtutils/string.lua:792: bad argument #1 to 'match' (string expected, got nil)
stack traceback:
	[C]: in ?
	[C]: in function 'string.match'
	[HOME]/.config/darktable/lua/lib/dtutils/string.lua:792: in function 'lib/dtutils.string.build_substitute_list'
	.../.config/darktable/lua/contrib/rename_images.lua:144: in upvalue 'do_rename'
	.../.config/darktable/lua/contrib/rename_images.lua:211: in function <.../.config/darktable/lua/contrib/rename_images.lua:210>
	[C]: in ?

No matter what pattern I define or if am trying to rename JPEG or ARW file, I am getting the error above.

[wpferguson]

What do the images you are trying to rename have for date time taken?

[deekayhd]

@wpferguson I think this is the same bug as reported on pixls.us (https://discuss.pixls.us/t/rename-within-darktable/26744/69)
This came in with PR #653. Variable datetime_taken in line 788 and 792 of strings.lua is not initialized.
I will provide a PR to fix it. Sorry for that.

[deekayhd]

PR is ready.