tmf: Mark ITmfSourceLookup#getCallsite as @Nullable

tmf: Mark ITmfSourceLookup#getCallsite as @Nullable

Any event that implements ITmfSourceLookup may return null to
indicate there is no specific call site information for this
event.

Clarify this by marking the method @Nullable, which ensures this
case is handled gracefully.

Change-Id: I2c38e879bea8d90fb3a19aa7d16f6d6566cccb2c
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/50311
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>