Fix WebUI Admin Action infinite retry with no MFA devices

[Joerger]

#49679 and subsequently #50570 introduced a change where getMfaChallengeResponse could return null | undefined when the user had no MFA challenges (no devices or not required). fetchJsonWithMfaAuthnRetry expects getMfaChallengeResponse to return {} in these cases, and will retry with MFA until it gets either an empty or non-empty object.

This PR fixes the issue by:

1. addressing the recursive fetchJsonWithMfaAuthnRetry which could result in an infinite loop. It's no longer recursive so it will only retry once even if null or undefined is received.
2. Reverting part of https://github.com/gravitational/teleport/pull/50570/files and instead making getMfaChallengeResponse return {} so we can properly determine at any point whether an mfa response is undefined or an empty response resulting from a no-op challenge attempt (no devices or not required).

Closes #51105