Bug 1381876 - Allow assert_success to be used without an expected value. r=automatedtester

It happens that we receive responses that do not have a body value.
We want to be able to use assert_success with these as well.

MozReview-Commit-ID: B1f0Hn406Nj

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extra : rebase_source : 6f47e4bd8a72fecf365a6e06771effb1f8e055a5