fix: small windows no longer constantly throw errors

[hankertrix]

Fixes #827 and #1759

The entries_win:open function will fail when either the width or the height of the window is less than 1, which will result in the nvim_win_set_cursor being passed a nil value for self.entries_win.win instead of a number, which causes an error to constantly appear when the window is small. Hence, a guard clause to check for whether self.entries_win.win is nil is added to stop the error from occurring.

Also fixed the rarer 'height' must be a positive Integer error caused by the window.update function, within the clause to draw the scrollbar background. This error happens on small windows when pressing and holding down the backspace key to delete a lot of characters at once. The nvim_open_win function call to create the scrollbar background throws the error when self.style.height is 0. Hence, an additional check is added alongside the info.scrollable check which also skips drawing the scrollbar thumb as it is not needed when the height is 0 and will result in a weird scrollbar thumb floating some distance away from the text when holding down backspace to delete a lot of characters.

[hrsh7th]

Thank you!