textbuffer: unit tests for the empty last line

For functions using _gtk_text_buffer_get_line_log_attrs():
- gtk_text_buffer_backspace()
- some gtk_text_iter functions (word/sentence/cursor boundaries)

As the FIXME comments show, there is a bug with
gtk_text_iter_is_cursor_position() for an empty last line.

https://bugzilla.gnome.org/show_bug.cgi?id=156164
Simplify _gtk_text_buffer_get_line_log_attrs()

NULL was returned in case of an empty last line. Every users needed to
special-case this. Now it will return the expected result: char_len of 0
with one PangoLogAttr.

In compute_log_attrs(), 'paragraph' will be the empty string "" with
'char_len' == 0.
pango_get_log_attrs() works fine with an empty string, it will return
one correct PangoLogAttr (because there is one text position for the
empty string).

It fixes the unit tests for gtk_text_iter_is_cursor_position().

https://bugzilla.gnome.org/show_bug.cgi?id=156164