fix(move): no-op when buffer has no treesitter parser (#883)

[simonyang08]

Fixes #883.

Movement functions (goto_next_start, goto_next_end, goto_previous_start, goto_previous_end, goto_next, goto_previous, and the repeatable_move wrappers) raised a Lua error on buffers without an attached treesitter parser:

E5108: .../move.lua:86: attempt to perform arithmetic on local 'score' (a nil value)
E5108: .../move.lua:142: attempt to compare two nil values

Root cause

shared.find_best_range() returned an empty table ({}) when no parser was attached. An empty table is truthy in Lua, so move.lua proceeded to call scoring_function({}) and filter_function({}), dereferencing nil fields.

select_textobject was already silent because best_range_at_point() returns nil when its input list is empty — the asymmetry is purely on the find_best_range side.

Fix

Make find_best_range() return nil (matching its annotated return type Range6?) instead of {} when the parser is missing. The existing if current_range then guard in move.lua then correctly skips the iteration, and goto_node(nil, …) already no-ops in that case.

Test

New tests/move/no_parser_spec.lua: renders a scratch buffer with no filetype/parser and asserts every movement entry point is a silent no-op. The spec fails on main (9 of the calls raise the two error shapes from the issue) and passes with the fix.

Note: on buffers whose filetype maps to a registered but uninstalled parser, vim.treesitter.get_parser itself can still throw — that is core behavior outside this fix.

stylua clean; full local suite not run end-to-end (the bundled nvim-treesitter install script fails in this environment, unrelated to this change) — CI will exercise it.