[SYCL] Don't return empty platforms

[cperkinsintel]

collecting empty platforms leads to confusion for things like platform.has(aspect)

[smaslov-intel]

Does spec mandate (or even allow) that platforms with no devices be filtered?
A test that I am adding for UR wants the empty platform be reported: intel/llvm-test-suite#1450

[cperkinsintel]

/verify with intel/llvm-test-suite#1491

[againull]

Probably we should report that empty platform doesn't have any device aspects, because it doesn't have any devices.
Now we report that empty platform has any aspect which seems like a mistake. @smaslov-intel , @cperkinsintel what do you think?
In this case we still can expose empty platforms through get_platforms().

[smaslov-intel]

Probably we should report that empty platform doesn't have any device aspects, because it doesn't have any devices. Now we report that empty platform has any aspect which seems like a mistake. @smaslov-intel , @cperkinsintel what do you think? In this case we still can expose empty platforms through get_platforms().

With this change empty platforms are not reported, so there is no way to query aspects from such.

[cperkinsintel]

Matching update to test suite here: intel/llvm-test-suite#1493

[bader]

Matching update to test suite here: intel/llvm-test-suite#1493

Please, test them together.

[cperkinsintel]

/verify with intel/llvm-test-suite#1493