[CSL-1794] Permit absence of `ProposeUpdateSyste' in `propose' command
[CSL-1794] Permit absence of `ProposeUpdateSyste' in `propose' command

Nowadays it's perfectly valid to have an update proposal with empty `upData'.

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl