fix(auth): Prevent crash if KeyStore not available (ex: instant app)

[tylerjroach]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:
#2991

Description of changes:
Prevents uncaught exception that occurs in legacy keystore migration when a keystore isn't available (ex: in an Instant App Launch). Verified that Auth + Authenticator will allow in memory sign in's with these changes.

How did you test these changes?
(Please add a line here how the changes were tested)

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.