RescuedExceptionInterceptor: Handle empty configuration

[MrSerth]

Previously, it could happen that Sentry.configuration was nil. In this case, calling rails would produce a NoMethodError. We fix this issue by using safe navigation.

Furthermore, this commit ensures we use a reasonable default in case the configuration couldn't be loaded. Since the config report_rescued_exceptions defaults to true, we assume this value here, too.

Fixes #2386