Improve branch coverage testing for base 64, specifically around preconditions

[ahsonkhan]

We are at ~80% branch coverage, which could be improved to get closer to ~100%

https://dev.azure.com/azure-sdk/public/_build/results?buildId=990064&_a=summary&tab=ms.vss-codecoverage-web.code-coverage-details&view=codecoverage-tab

https://github.com/Azure/azure-sdk-for-c/blob/d9ef1bf5c5bdc775fcf5a86f144d27a272054a49/sdk/src/azure/core/az_base64.c#L319-L324

See examples on how to add tests for preconditions here: https://github.com/Azure/azure-sdk-for-c/blob/9f5a6334863ebbf214c6e2fbd5d6e3fce52fe0de/sdk/tests/iot/hub/test_az_iot_hub_client_c2d.c#L41-L54