fix(@cockpit): don't send invalid value to Smart Contract methods

We've introduced a regression in 536a402 where the default
`value` sent to payable Smart Contract methods is `-1`, resulting in errors as it's not
a valid value.

packages/embark-ui/src/components/ContractOverview.js

@@ -37,7 +37,7 @@ class ContractFunction extends Component {
       optionsCollapse: false,
       functionCollapse: false,
       gasPriceCollapse: false,
-      value: -1,
+      value: 0,
       unitConvertError: null,
       unitConvertDirty: false
     };