patch 8.2.1804: resolve('/') returns an empty string

Problem:    resolve('/') returns an empty string.
Solution:   Don't remove single slash. (closes #7074)

src/filepath.c

@@ -1889,7 +1889,7 @@ f_resolve(typval_T *argvars, typval_T *rettv)
 	    is_relative_to_current = TRUE;
 
 	len = STRLEN(p);
-	if (len > 0 && after_pathsep(p, p + len))
+	if (len > 1 && after_pathsep(p, p + len))
 	{
 	    has_trailing_pathsep = TRUE;
 	    p[len - 1] = NUL; // the trailing slash breaks readlink()

src/testdir/test_functions.vim

@@ -339,6 +339,8 @@ func Test_resolve_unix()
   call assert_equal('Xlink2', resolve('Xlink1'))
   call assert_equal('./Xlink2', resolve('./Xlink1'))
   call delete('Xlink1')
+
+  call assert_equal('/', resolve('/'))
 endfunc
 
 func s:normalize_fname(fname)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1804,
 /**/
     1803,
 /**/