Indent settings not detected on files without a filetype

[bshimanuki]

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vim-sleuth fails to detect indent settings in a file that does not have a filetype (eg, a file with a custom extension).

Expected Behavior: Detect indent settings based on the current file. Either skip siblings/parent directories or look at other files with the same extension (if it has an extension).

Actual Behavior: The DetectHeuristics short circuits on https://github.com/tpope/vim-sleuth/blob/1cc4557420f215d02c4d2645a748a816c220e99b/plugin/sleuth.vim#L520 due to the empty filetype check