patch 8.2.3939: MS-Windows: fnamemodify('', ':p') does not work

Problem:    MS-Windows: fnamemodify('', ':p') does not work.
Solution:   Do not consider an empty string a full path. (Yegappan Lakshmanan,
            closes #9428, closes #9427)
diff --git a/src/testdir/test_fnamemodify.vim b/src/testdir/test_fnamemodify.vim
index dfe322a..c19f464 100644
--- a/src/testdir/test_fnamemodify.vim
+++ b/src/testdir/test_fnamemodify.vim
@@ -32,6 +32,7 @@
   call assert_equal('fb2.tar.gz', fnamemodify('abc.fb2.tar.gz', ':e:e:e'))
   call assert_equal('fb2.tar.gz', fnamemodify('abc.fb2.tar.gz', ':e:e:e:e'))
   call assert_equal('tar', fnamemodify('abc.fb2.tar.gz', ':e:e:r'))
+  call assert_equal(getcwd(), fnamemodify('', ':p:h'))
 
   let cwd = getcwd()
   call chdir($HOME)