Fix #12623
problem: nvim_buf_get_lines(0) returns empty during startup, where no buffers are loaded yet.
solution: return empty object
Happens during startup, where buffer may not be loaded yet, because...
`source_startup_scripts()` is done before `edit_buffers()`:
9bb552875d/src/nvim/main.c (L362)9bb552875d/src/nvim/main.c (L480)