patch 8.2.4842: expand("%:p") is not empty when there is no buffer name

Problem:    expand("%:p") is not empty when there is no buffer name.
Solution:   When ignoring errors still return NULL. (closes #10311)

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4842,
 /**/
     4841,
 /**/