[CSL-1179] Use 'stopProperty' instead of 'assert' in 'verifyEmptyMainBlock'
[CSL-1179] Use 'stopProperty' instead of 'assert' in 'verifyEmptyMainBlock'

It allows us to see the message when verification fails.

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