patch 9.0.0718: extra empty line between two virtual text "below"

Problem:    Extra empty line between two virtual text "below" when 'wrap' and
            'number' are set.
Solution:   Reset "before" when there is no text in the screen line.
            (closes #11334)

src/testdir/test_textprop.vim

@@ -2653,6 +2653,9 @@ func Test_props_with_text_after_below_trunc()
   let buf = RunVimInTerminal('-S XscriptPropsAfterTrunc', #{rows: 8, cols: 60})
   call VerifyScreenDump(buf, 'Test_prop_with_text_after_below_trunc_1', {})
 
+  call term_sendkeys(buf, ":set number\<CR>")
+  call VerifyScreenDump(buf, 'Test_prop_with_text_after_below_trunc_2', {})
+
   call StopVimInTerminal(buf)
 endfunc