[Locale] Fixed: StubLocale::setDefault() throws no exception when "en" is passed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This is necessary to fix `IntlTestHelper::requireIntl()` when ICU is not installed and the Icu component 1.0.0 is used instead. In that case, `IntlTestHelper::requireIntl()` calls `\Locale::setDefault('en')` and fails. Consequently, the Form component test cases are partially red when running without ICU installed.