Fixing slice construction for uninitialized AoSoAs

[sslattery]

A bug was discovered in #79 where slices created from uninitialized AoSoAs would try to dereference the first value of the AoSoA data. Now we check to see if there is any data. If there is we dereference it. Otherwise we build the slice on a raw pointer. This is valid because the slice is given a corresponding size of 0. A user should not dereference data in a slice of size 0.

[codecov-io]

Codecov Report

Merging #80 into master will increase coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master     #80     +/-   ##
========================================
+ Coverage    97.7%   97.7%   +<.1%     
========================================
  Files          17      17             
  Lines        1097    1100      +3     
========================================
+ Hits         1072    1075      +3     
  Misses         25      25

Impacted Files	Coverage Δ
core/src/Cabana_AoSoA.hpp	100% <100%> (ø)	⬆️

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