api: Fix clang warning

api: Fix clang warning

Fix the following clang warnings by initializing the char pointers to
NULL rather than '\0';

	api.c:4232:46: warning: expression which evaluates to zero treated as a
	null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
	 4232 |         char *controller_list[MAX_MNT_ELEMENTS] = { '\0' };
	      |                                                     ^~~~
	api.c:4233:40: warning: expression which evaluates to zero treated as a
	null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
	 4233 |         char *cgrp_list[MAX_MNT_ELEMENTS] = { '\0' };
	      |                                               ^~~~

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>