Hide converted price of token if no conversion rate found

[danjm]

Currently on the send screen, if we do not have currency conversion data for a token, we show represent the converted amount as 0 in whatever the users selected currency is. For example, see the '$0.00 USD' in the screenshots of #4645

If we do not have currency conversion information, we simply should not show a converted currency.

[danjm]

To replicate:

• switch to ropsten network
• go to https://faucet.metamask.io/ and request 1 ether from faucet
• go to https://tokenfactory.surge.sh/#/ and create token contract (e.g. Enter: 100, ABC, 0, ABC to keep things simple)
• once the token is created, copy the contract address
• back in metamask, click "Add Token"
• click the "Custom Token" tab and copy the address of the token you just created, the other fields should autopopulate
• click Next and Next
• select the newly created token in the sidebar
• click 'Send'

You will see the above describe issue on the send screen

[snreynolds]

Closed with #4777