[Bug]: inconsistency in representation for unavailable fiat conversion

**Describe the bug**

Inconsistency in representation for unavailable fiat conversion. In the token hero, we show it as `$0`, but within gas modal, we show it as `--`

**Screenshots/Recordings**

**Steps to reproduce**

1. Go to test dapp
2. Trigger a token transfer
3. Notice how fiat values are displayed in different components

**Error messages or log output**

```shell

```

**Detection stage**

in production

**Version**

7.53.0

**Build type**

None

**Device**

iphone 11

**Operating system**

iOS

Start by reproducing the discrepancy in the test dapp during a token transfer, then trace how unavailable fiat values are formatted in the token hero and gas modal. Done means both components consistently represent an unavailable conversion instead of showing `$0` in one place and `--` in the other.

Written by the indexing model from the issue text.