Fix NPE in pg_regress

Author: maksm90

src/test/regress/pg_regress.c

@@ -74,7 +74,7 @@ char	   *inputdir = ".";
 char	   *outputdir = ".";
 char	   *bindir = PGBINDIR;
 char	   *launcher = NULL;
-char	   *xid_options = NULL;
+char	   *xid_options = "";
 static _stringlist *loadlanguage = NULL;
 static _stringlist *loadextension = NULL;
 static int	max_connections = 0;