alpaka: `static_assert(Dim::value >= 1)` fails in pixeltrack

Since this commit pixeltrack-standalone fails to compile because of:

/afs/cern.ch/user/a/aperego/public/pixeltrack-standalone/external/alpaka/include/alpaka/extent/Traits.hpp(92): error: static assertion failed
          detected during:
            instantiation of "auto alpaka::getWidth(const TExtent &)->alpaka::Idx<TExtent> [with TExtent=alpaka_common::Scalar]"
/afs/cern.ch/user/a/aperego/public/pixeltrack-standalone/src/alpaka/AlpakaCore/CachedBufAlloc.h(66): here
            instantiation of "auto cms::alpakatools::traits::CachedBufAlloc<TElem, TDim, TIdx, alpaka::DevCudaRt, TQueue, void>::allocCachedBuf(const alpaka::DevCudaRt &, TQueue, const TExtent &)->alpaka::BufCudaRt<TElem, TDim, TIdx> [with TElem=ZVertexSoA, TDim=alpaka_common::Dim0D, TIdx=alpaka_common::Idx, TQueue=alpaka_cuda_async::Queue, TExtent=alpaka_common::Scalar]"
/afs/cern.ch/user/a/aperego/public/pixeltrack-standalone/src/alpaka/AlpakaCore/memory.h(193): here
            instantiation of "std::enable_if_t<<expression>, cms::alpakatools::device_buffer<alpaka::Dev<TQueue>, T>> cms::alpakatools::make_device_buffer<T,TQueue>(const TQueue &) [with T=ZVertexSoA, TQueue=alpaka_cuda_async::Queue]"
/afs/cern.ch/user/a/aperego/public/pixeltrack-standalone/src/alpaka/plugin-PixelVertexFinding/alpaka/gpuVertexFinder.cc(113): here

to reproduce:

git clone https://github.com/AuroraPerego/pixeltrack-standalone.git -b newBuf
cd pixeltrack-standalone && git checkout 9956d02bc3bbfc0acdd06849fbbbda997863538b
make environment
. env.sh
make alpaka

FYI @bernhardmgruber