Update FUIAuth.m

[kbot]

Prevent an error from being swallowed in the case of FIRAuthErrorCodeAccountExistsWithDifferentCredential when the emailAiuthProvider is nil.

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[morganchen12]

Hey @kbot, thanks for the contribution! Please sign the CLA, then we'll have someone review this PR.

[kbot]

@googlebot I signed it!

[morganchen12]

Thanks!