Clean aws_cryptosdk_cmm_generate_enc_materials preconditions

`aws_cryptosdk_cmm_generate_enc_materials` has tests which pass invalid input, and check for the expected error code. These tests pass input which is invalid in several params, but only expects this to matter for the one they're checking. I had to allow `NULL` inputs for params that really shouldn't be to get those tests to pass. A better solution would be to either
1. Fix the tests
2. confirm that these are really the preconditions we want.

Start by locating aws_cryptosdk_cmm_generate_enc_materials and its tests. Review each test's invalid parameter combinations and compare them with the function's intended preconditions and returned error codes. Done means the tests independently validate the parameter under test without relying on unrelated invalid inputs, and the preconditions are confirmed or corrected.

Written by the indexing model from the issue text.