Generic SSZ encode/decode issues on general 1.5.0-beta.3 spec tests

[rodrigo-o]

This appears to be unrelated to the electra update and instead being something now tested in the phase0 general spec tests that fails on a particular parameter (spectest version 1.5.0-beta.3):

1) test c:general f:phase0 r:ssz_generic h:containers s:valid -> ComplexTestStruct_nil_8 (General.Phase0.SszGenericTest)
     test/generated/general/phase0/ssz_generic.exs:9689
     Assertion with == failed
     code:  assert deserialized == real_deserialized
     left:  %Helpers.SszStaticContainers.ComplexTestStruct{
              A: 9027,
              B: [],
              C: 140,
              D: [],
              E: %Helpers.SszStaticContainers.VarTestStruct{A: 54691, B: [], C: 202},
    [...]
    right: %Helpers.SszStaticContainers.ComplexTestStruct{
              A: 9027,
              B: [],
              C: 140,
              D: [0],
              E: %Helpers.SszStaticContainers.VarTestStruct{A: 54691, B: [], C: 202},

This can be easily replicated on the electra-support branch running make spec-test-general-*.

14 test fail equally on that suite and they all have the same difference: [] instead of [0].

[rodrigo-o]

Closed in #1400