Fix display of indicate-empty-lines when line numbers are displayed

Fix display of indicate-empty-lines when line numbers are displayed

* src/xdisp.c (row_text_area_empty): New function.
(display_line): Call row_text_area_empty to verify that a glyph
row's text area is devoid of any glyphs that came from a buffer or
a string.  This fixes a bug with empty-lines indication
disappearing when line numbers or line-prefix are displayed.
(display_line): Delete the argument FORCE; all callers changed.
Remove the condition for actually producing the glyphs for the
line number, as even if the number didn't change we need to
produce empty space.