patch 9.1.1489: terminal: no visual highlight of empty cols with empty 'listchars'

Problem:  terminal: no visual highlight of empty cols when 'listchars'
          is empty (Yousef Mohammed)
Solution: reset wlv.win_attr when column is zero and we are drawing a
          terminal buffer

fixes: #17559
closes: #17618

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/drawline.c b/src/drawline.c
index 84a1d89..afc0480 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -2772,6 +2772,13 @@
 #endif
 		// no more cells to skip
 		skip_cells = 0;
+#ifdef FEAT_TERMINAL
+		if (term_show_buffer(wp->w_buffer)
+		    && wlv.col == 0
+		    && wlv.win_attr == term_get_attr(wp, lnum, -1))
+		    // reset highlighting attribute
+		    wlv.win_attr = 0;
+#endif
 	    }
 
 	    if (has_mbyte)