Allow odm_file_contexts to be empty

There might be an empty odm_file_contexts when BOARD_ODM_SEPOLICY_DIRS
is defined. Skip reading empty file_contexts files in function
compute_file_contexts_hash(), instead of returning an error, which makes
device doesn't boot.

Bug: 64240127
Test: boot a device having an empty odm_file_contexts

Change-Id: I359da5fce17eedb65ce288bb2781e61e8ade3cf9
1 file changed