[SDK-3350] Empty credentials before continuing should throw CredentialsManagerException

[poovamraj]

Changes

Trying to fetch the credentials JSON after it has been cleared was not throwing exception inside continueGetCredentials. We have added a CredentialsManagerException for this case.

This was missed out previously since continueGetCredentials would be called after the check for validCredentials in getCredentials method but this misses out 2 scenarios

• Credential being cleared before reaching continuingGetCredentials by another thread
• Credential being cleared before checkAuthenticationResult

References

GH Issue - #555

Testing

We have added a test case to our Unit test suite. We also did an integration test by the case mentioned in the GH issue

• This change adds unit test coverage

• This change adds integration test coverage

• This change has been tested on the latest version of the platform/language or why not