patch 8.0.0998: strange error when using K while only spaces are selected

Problem:    Strange error when using K while only spaces are selected.
            (Christian J. Robinson)
Solution:   Check for blank argument.
diff --git a/src/testdir/test_help.vim b/src/testdir/test_help.vim
index 5a35b69..85c1296 100644
--- a/src/testdir/test_help.vim
+++ b/src/testdir/test_help.vim
@@ -12,4 +12,21 @@
 func Test_help_errors()
   call assert_fails('help doesnotexist', 'E149:')
   call assert_fails('help!', 'E478:')
+
+  new
+  set keywordprg=:help
+  call setline(1, "   ")
+  call assert_fails('normal VK', 'E349:')
+  bwipe!
+endfunc
+
+func Test_help_keyword()
+  new
+  set keywordprg=:help
+  call setline(1, "  Visual ")
+  normal VK
+  call assert_match('^Visual mode', getline('.'))
+  call assert_equal('help', &ft)
+  close
+  bwipe!
 endfunc