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

azure-sdk-for-c/sdk/src/azure/core/az_base64.c

Lines 319 to 324 in d9ef1bf

	AZ_NODISCARD az_result
	az_base64_encode(az_span destination_base64_text, az_span source_bytes, int32_t* out_written)
	{
	_az_PRECONDITION_VALID_SPAN(destination_base64_text, 4, false);
	_az_PRECONDITION_VALID_SPAN(source_bytes, 1, false);
	_az_PRECONDITION_NOT_NULL(out_written);

See examples on how to add tests for preconditions here:

azure-sdk-for-c/sdk/tests/iot/hub/test_az_iot_hub_client_c2d.c

Lines 41 to 54 in 9f5a633

	#ifndef AZ_NO_PRECONDITION_CHECKING
	ENABLE_PRECONDITION_CHECK_TESTS()
	static void test_az_iot_hub_client_c2d_parse_received_topic_NULL_client_fail()
	{
	az_span received_topic = AZ_SPAN_FROM_STR(
	"devices/useragent_c/messages/devicebound/$.mid=79eadb01-bd0d-472d-bd35-ccb76e70eab8&$.to=/"
	"devices/useragent_c/messages/deviceBound&iothub-ack=full");
	az_iot_hub_client_c2d_request out_request;
	ASSERT_PRECONDITION_CHECKED(
	az_iot_hub_client_c2d_parse_received_topic(NULL, received_topic, &out_request));
	}

[github-actions]

Hi @ahsonkhan, we deeply appreciate your input into this project. Regrettably, this issue has remained inactive for over 2 years, leading us to the decision to close it. We've implemented this policy to maintain the relevance of our issue queue and facilitate easier navigation for new contributors. If you still believe this topic requires attention, please feel free to create a new issue, referencing this one. Thank you for your understanding and ongoing support.