api: Fix clang warning

api: Fix clang warning

Fix the following clang warning by using NULL instead of '\0'

	api.c:6721:17: warning: expression which evaluates to zero treated as a
	null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
	 6721 |         mnt_paths[i] = '\0';
	      |                        ^~~~

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