Fix custom nonce placeholder type (#7243)

The placeholder for the custom nonce needed to be converted into a
string. The placeholder is omitted if `nextNonce` isn't set, as may be
the case for the initial render.

ui/app/pages/confirm-transaction-base/confirm-transaction-base.component.js

@@ -282,7 +282,7 @@ export default class ConfirmTransactionBase extends Component {
                 <TextField
                   type="number"
                   min="0"
-                  placeholder={ nextNonce }
+                  placeholder={ nextNonce ? nextNonce.toString() : null }
                   onChange={({ target: { value } }) => {
                     if (!value.length || Number(value) < 0) {
                       updateCustomNonce('')