Correct return value of adios2_variable_ndims
Correct return value of adios2_variable_ndims

For local arrays, `m_Shape` is empty, while `m_Count` is not, so we read off ndims there.

Closes https://github.com/ornladios/ADIOS2/issues/2711
ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2