fix(github) Fix proptype warning in github issue link (#10511)
fix(github) Fix proptype warning in github issue link (#10511)

The externaIssue identifier autocomplete had no default choices and
React was emitting the following console error:

```
Warning: Failed prop type: The prop `choices` is marked as required in `SelectField`, but its value is `undefined`.
   in SelectField (created by FieldFromConfig)
   in FieldFromConfig (created by ExternalIssueForm)
   in div (created by Form)
```

Setting a default value fixes this problem.
mvaled/sentry,beeftornado/sentry,mvaled/sentry,mvaled/sentry,mvaled/sentry,mvaled/sentry,beeftornado/sentry,beeftornado/sentry,mvaled/sentry