skip md5 check if hash value is empty

[thinhbuzz]

save the cost of checking md5 when the hash is empty.

[azhon]

I think check if hash value empty is unnecessary, finally FileUtil.md5(file).equals(manager.apkMD5, ignoreCase = true) always return false.

[thinhbuzz]

@azhon in the current behavior we still need the cost to generate the md5 hash for comparison. with this PR we don't need to generate md5 hash

[azhon]

@thinhbuzz sure. add this PR in the next version