lttng.ust: Properly null-check the debug info state system

lttng.ust: Properly null-check the debug info state system

When opening a trace, some views (callstack etc.) might request
binaries before the state system is properly loaded,
so the checkNotNull() check can fail. Actually reply that the
information is not available, instead of breaking the whole trace.

Change-Id: Iba14ac2ffac6f01d45bba4dba9f946372777b5db
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/76255
Reviewed-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Tested-by: Alexandre Montplaisir <alexmonthy@efficios.com>