patch 8.2.3337: completing "call g:" returns entries with just "g:"

Problem:    Completing "call g:" returns entries with just "g:". (Naohiro Ono)
Solution:   Skip empty strings returned by get_user_func_name(). (closes #8753)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index b90c6d7..2b50798 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2304,7 +2304,7 @@
     if (intidx < 0)
     {
 	name = get_user_func_name(xp, idx);
-	if (name != NULL)
+	if (name != NULL && *name != NUL)
 	{
 	    if (*name != '<' && STRNCMP("g:", xp->xp_pattern, 2) == 0)
 		return cat_prefix_varname('g', name);