fix(drawline): don't invoke on_line for filler line (#28219)

Problem:
Decoration provider `on_line` handler is invoked for diff filler line
below the last buffer line. This does not match the documentation:
"called for each buffer line".

Solution:
Check `end_fill`.

src/nvim/drawline.c

@@ -1030,7 +1030,9 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int col_rows, s
 
     has_decor = decor_redraw_line(wp, lnum - 1, &decor_state);
 
-    decor_providers_invoke_line(wp, lnum - 1, &has_decor);
+    if (!end_fill) {
+      decor_providers_invoke_line(wp, lnum - 1, &has_decor);
+    }
 
     if (has_decor) {
       extra_check = true;