CLI: Only show magnetizations in case they are present for --print-table

[mbercx]

When using the --print-table option for the plot commands, the Total magnetization column is shown even in case a non spin-polarized calculation was run, and hence no magnetizations are in the outputs:

$ aiida-common-workflows plot eos 9703 --print-table
  Volume (Å^3)    Energy (eV)  Total magnetization (μB)
--------------  -------------  --------------------------
       61.9925       -2149.73
       63.3115       -2149.76
       64.6305       -2149.78
       65.9494       -2149.79
       67.2684       -2149.79
       68.5874       -2149.78
       69.9064       -2149.76

In this case, I would remove this column since it has no content.