ss: Mark ITmfStateSystem#queryOngoingState as @NonNull

ss: Mark ITmfStateSystem#queryOngoingState as @NonNull

Just like querySingleState, an ongoing state cannot be null
if the passed attribute is valid. At worst the value can be a
NullStateValue.

Change-Id: I138ce6d0d62a0cb45706117fd4d0e4bdf8e8fc58
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/68815
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>