commit #3d667c32d3ac04 fixed a bug, that a section was not considered
empty for the statusline, also g:airline_skip_empty was set.
However unfortunately, this lead to a regression, makeing the tabline
ugly, because sections, that contained a single highlighting group would
be considered empty and would therefore be skipped. Since this is not
what is expected, make s:section_is_empty() return zero, when it notices
we are looking at a tabline.
fixes#1273
f5359f9177