patch 8.2.3939: MS-Windows: fnamemodify('', ':p') does not work

Problem:    MS-Windows: fnamemodify('', ':p') does not work.
Solution:   Do not consider an empty string a full path. (Yegappan Lakshmanan,
            closes #9428, closes #9427)
diff --git a/src/version.c b/src/version.c
index 15df860..0f0414e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3939,
+/**/
     3938,
 /**/
     3937,