patch 8.1.1924: using empty string for current buffer is unexpected

Problem:    Using empty string for current buffer is unexpected.
Solution:   Make the argument optional for bufname() and bufnr().

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1924,
 /**/
     1923,
 /**/