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().
diff --git a/src/version.c b/src/version.c
index 8ae3dda..cab79da 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1924,
+/**/
     1923,
 /**/
     1922,