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

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

src/version.c

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