patch 9.0.1301: virtual text below empty line not displayed

Problem:    Virtual text below empty line not displayed.
Solution:   Adjust flags and computations. (closes #11959)
diff --git a/src/testdir/dumps/Test_prop_above_below_empty_1.dump b/src/testdir/dumps/Test_prop_above_below_empty_1.dump
new file mode 100644
index 0000000..f470016
--- /dev/null
+++ b/src/testdir/dumps/Test_prop_above_below_empty_1.dump
@@ -0,0 +1,16 @@
+| +0#af5f00255#ffffff0@3|-+0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@1|1| |1+0#0000000&@7| @47
+| +0#af5f00255&@3|++0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@3|-+0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@1|2| | +0#0000000&@55
+| +0#af5f00255&@3|++0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@3|-+0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@1|3| |3+0#0000000&@8| @46
+| +0#af5f00255&@3|++0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@3|-+0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@1|4| | +0#0000000&@55
+| +0#af5f00255&@3|++0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@3|-+0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+| +0#af5f00255&@1|5| >5+0#0000000&@10| @44
+| +0#af5f00255&@3|++0#0000001#ffff4012@2| +0#0000000#ffffff0@52
+@42|5|,|1|-|5|7| @7|A|l@1| 
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index e00bad6..b628c82 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -2779,6 +2779,28 @@
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_prop_with_text_above_below_empty()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      setlocal number
+      call setline(1, ['11111111', '', '333333333', '', '55555555555'])
+
+      let vt = 'test'
+      call prop_type_add(vt, {'highlight': 'ToDo'})
+      for ln in range(1, line('$'))
+        call prop_add(ln, 0, {'type': vt, 'text': '---', 'text_align': 'above'})
+        call prop_add(ln, 0, {'type': vt, 'text': '+++', 'text_align': 'below'})
+      endfor
+      normal G
+  END
+  call writefile(lines, 'XscriptPropAboveBelowEmpty', 'D')
+  let buf = RunVimInTerminal('-S XscriptPropAboveBelowEmpty', #{rows: 16, cols: 60})
+  call VerifyScreenDump(buf, 'Test_prop_above_below_empty_1', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_prop_with_text_below_after_match()
   CheckRunVimInTerminal