FixedOffsetZone constructor accepts bad inputs, but `isValid` returns true

FixedOffsetZone constructor accepts bad inputs, but `isValid` returns true

**Describe the bug**
When I set timezone with a ISO string as 'America/Chicago', the DateTime was created properly.
When I set timezone with a FixedOffsetZone object, the DateTime was created invalid.

**To Reproduce**
Please share a minimal code example that triggers the problem:
```
> var zone = new luxon.FixedOffsetZone('CDT');
undefined
> zone.isValid
true
> var d1 = luxon.DateTime.fromISO('2021-11-03T13:00:00', { zone: 'America/Chicago'});
undefined
> d1
DateTime {
  ts: 1635962400000,
  _zone: IANAZone { zoneName: 'America/Chicago', valid: true },
  loc: Locale {
    locale: 'en-US',
    numberingSystem: null,
    outputCalendar: null,
    intl: 'en-US',
    weekdaysCache: { format: {}, standalone: {} },
    monthsCache: { format: {}, standalone: {} },
    meridiemCache: null,
    eraCache: {},
    specifiedLocale: null,
    fastNumbersCached: null
  },
  invalid: null,
  weekData: null,
  c: {
    year: 2021,
    month: 11,
    day: 3,
    hour: 13,
    minute: 0,
    second: 0,
    millisecond: 0
  },
  o: -300,
  isLuxonDateTime: true
}
> var d2 = luxon.DateTime.fromISO('2021-11-03T13:00:00', { zone: zone });
undefined
> d2
DateTime {
  ts: NaN,
  _zone: FixedOffsetZone { fixed: 'CDT' },
  loc: Locale {
    locale: 'en-US',
    numberingSystem: null,
    outputCalendar: null,
    intl: 'en-US',
    weekdaysCache: { format: {}, standalone: {} },
    monthsCache: { format: {}, standalone: {} },
    meridiemCache: null,
    eraCache: {},
    specifiedLocale: null,
    fastNumbersCached: null
  },
  invalid: Invalid { reason: 'invalid input', explanation: undefined },
  weekData: null,
  c: null,
  o: null,
  isLuxonDateTime: true
}
```

**Actual vs Expected behavior**
I expect that d2 is valid because the timzone is valid and CDT and America/Chicago is basically the same.

**Desktop (please complete the following information):**

- OS: mac
- Browser: N/A
- Luxon version: 2.0.2
- Your timezone: America/Los Angeles

**Additional context**
Add any other context about the problem here.