Bug in LayerCreator when layer has no surfaces (was ACTS-435)

Original author Julia Hrdinka @jhrdinka

After the recent updates of surface array creator and the layer creator we get a segmentation fault in case a layer has no surfaces (material layer) [here](https://gitlab.cern.ch/acts/acts-core/blob/master/Core/src/Tools/LayerCreator.cpp#L379).

Therefore before trying to access the surfaces it should be checked, if the layer has a surfaceArray (not nullptr).

Edited Mar 27, 2019 by Moritz Kiehn

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