fix(border): only set border to none if winborder doesn't exist

[seblyng]

Problem: Custom border characters in winborder was just added. It needs to be a list of characters. vim.o.winbar gives the string value with comma separated items which is invalid.

Solution: Just never explicitly set the border if winborder option exists. From the docs of winborder:

Defines the default border style of floating windows. The default value
is empty, which is equivalent to "none".