Improve tree-sitter's prev-sibling indent anchor

Improve tree-sitter's prev-sibling indent anchor

Now it handles the case where NODE is nil when indenting an empty
line: it tries to get the previous sibling nonetheless.

* lisp/progmodes/c-ts-mode.el (c-ts-mode--anchor-prev-sibling):
* lisp/treesit.el (treesit-simple-indent-presets): Add an or form to
handle more cases.