[Spec change] Fix contradiction with context constructors

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#628 by @gmlueck requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue.
If no changes are required: Close this issue and proceed with the spec PR.

[gmlueck]

The CTS already has tests for most of the context constructors in KhronosGroup/SYCL-Docs#628, but we need new CTS tests for these overloads:

• context(const platform &plt, const property_list &propList = {})
• context(const platform &plt, async_handler asyncHandler, const property_list &propList = {})

There are no CTS tests the check for the case when the device list is empty. However, the CTS generally does not test error conditions, so it is not necessary to add these tests.