ext4_utils: fix build error without FILE_CONTEXTS

After commit 4df62f342dbbe2f5cca831ce789dc0426d32ec03,
occurs segmentation fault if mkuserimg.sh runs without FILE_CONTEXTS options.
It's due to strlen() for null value. To fix it, return 0 for null.

Change-Id: Icf99706e51b9053a24e23b9d081a258962877c30
1 file changed