h5netcdf: Don't assume there are dimensions when there are chunks

[rho-novatron]

• Closes With backend h5netcdf and driver h5pyd there are cases when len(var.chunks) > 0 but len(var.dimensions) == 0 #10091
• Tests added - NO – I don't know how this should mock the h5netcdf library
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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