Constant Records for JoinedArrays: Skip `shape` for Particles

[ax3l]

For ADIOS2 JoinedArray support, we need to address this regression:
openPMD/openPMD-standard#289

Opening this issue to document our discussion results two weeks ago @franzpoeschel @guj @ax3l

[franzpoeschel]

Moving this to 0.17 since the associated PR #1661 is also part of the 0.17 milestone.