prepend portion of asset-urls not accounted for

Related https://github.com/ember-cli/ember-cli-terser/issues/275#issuecomment-1428987522

When specifying

```ts
new EmberApp(defaults, {
fingerprint: {
prepend: '<>'
}
})
```

It appears to be impossible to build with `ember-cli-terser` enabled at all, even when deactivating sourcemaps via

```ts
new EmberApp(defaults, {
sourcemaps: {
enabled: false
}
})
```

Just in case `ember-cli-terser` and `broccoli-terser-sourcemap` need to be aware of this URL I added the `publicUrl` option

```ts
new EmberApp(defaults, {
'ember-cli-terser': {
publicUrl: '<>'
},
fingerprint: {
enabled: false,
publicUrl: '<>'
}
})
```

All of this was to no avail. Even with sourcemaps disabled I get logs like the following:

```
[WARN] (broccoli-terser-sourcemap) "<>/assets/leaflet/leaflet-src-18322c41aab5808cecb4784380d74637.js.map" referenced in "assets/leaflet/leaflet-src-18322c41aab5808cecb4784380d74637.js" could not be found
```

As well as a build error:

```
Unexpected token punc «:», expected punc «,»
```

Using a debugger I find that this error is from within a sourcemap being parsed (surprising with them disabled?!)