patch 8.2.0018: :join does not add white space where it should

Problem:    :join does not add white space where it should. (Zdenek Dohnal)
Solution:   Handle joining multiple lines propely.
diff --git a/src/testdir/test_join.vim b/src/testdir/test_join.vim
index 5801c2f..377edab 100644
--- a/src/testdir/test_join.vim
+++ b/src/testdir/test_join.vim
@@ -427,3 +427,14 @@
   call assert_equal(expected, getline(1, '$'))
   close!
 endfunc
+
+func Test_join_lines()
+  new
+  call setline(1, ['a', 'b', '', 'c', 'd'])
+  %join
+  call assert_equal('a b c d', getline(1))
+  call setline(1, ['a', 'b', '', 'c', 'd'])
+  normal 5J
+  call assert_equal('a b c d', getline(1))
+  bwipe!
+endfunc