fix(document_symbols): add_blank_line_at_top

[nhat-vo]

Fix the bug when add_blank_line_at_top = true as mentioned in #879.

[MunifTanjim]

btw, add_blank_line_at_top can probably be supported by doing tree:render(2) instead of adding extra dummy nodes to the tree.

[cseickel]

Thanks for the heads up @MunifTanjim! We should probably do this at some point, but I tried to do it quickly and it didn't work out. There seems to be other code in neo-tree that assumes certain things about the number of nodes in the tree vs linenr. Also, any sort of re-render in the same window just added new copies of the tree below the old one.

If anyone else wants to take that on, go for it.