fix: respect tab count in `toggle_bufferline()`

[TommyCox]

Adjusts toggle_bufferline() to respect the number of tabs by falling back to builtin tab-count-based toggling of the tabline when always_show_bufferline = false and NOT in tabpage-only mode. Fixes #925.

[TommyCox]

@akinsho Just realized that the issue could be fixed by changing one character instead of rewriting multiple lines. The new commit just changes the fallback value from 0 (never show tabline) to 1 (show tabline when multiple tabpages exist).