runltp: Do not export empty LTP_BIG_DEV and LTP_BIG_DEV_FS_TYPE

Make runltp export LTP_BIG_DEV and LTP_BIG_DEV_FS_TYPE only when they
have a non-zero value. We do this to make getenv("LTP_BIG_DEV") or
getenv("LTP_BIG_DEV_FS_TYPE") return NULL, if user does not specify a
big block device. Because users usually write such code:

	device = getenv("LTP_BIG_DEV");
	if (device == NULL) {
		tst_brkm(TCONF, NULL,
			 "you must specify a big blockdevice(>1.2G)");
	} else {
		tst_mkfs(NULL, device, "ext3", NULL);
	}

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>