No error raised when a (non-zero) vector field is assigned with an empty list

[ericsson49]

When one assigns an empty list to a non zero vector (or initialized it with an empty list), the outcome of operation is ambiguous.
@protolambda has clarified that an error should be raised.
Currently, remerkleable.Vector doesn't raise an error.

Two code snippets illustrating that

>>> from eth2spec.phase1 import spec
>>> spec.Vector[spec.Bytes32,10]([])
Vector[SpecialByteVectorView, 10]<<len=10>>(
  0x0000000000000000000000000000000000000000000000000000000000000000,
  0x0000000000000000000000000000000000000000000000000000000000000000,
  0x0000000000000000000000000000000000000000000000000000000000000000,
  ... 4 omitted ...,
  0x0000000000000000000000000000000000000000000000000000000000000000,
  0x0000000000000000000000000000000000000000000000000000000000000000,
  0x0000000000000000000000000000000000000000000000000000000000000000
)

>>> from eth2spec.phase1 import spec
>>> spec.BeaconState().exposed_derived_secrets
Vector[List[ValidatorIndex, 32], 32768]<<len=32768>>(
  List[ValidatorIndex, 32]<<len=0>>(),
  List[ValidatorIndex, 32]<<len=0>>(),
  List[ValidatorIndex, 32]<<len=0>>(),
  ... 32762 omitted ...,
  List[ValidatorIndex, 32]<<len=0>>(),
  List[ValidatorIndex, 32]<<len=0>>(),
  List[ValidatorIndex, 32]<<len=0>>()
)
>>> spec.BeaconState().exposed_derived_secrets = []
>>>