fix: mark auth functions callbacks as optional

[mhagmajer]

Closes #857

[coveralls]

Pull Request Test Coverage Report for Build 6486104539

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.669%

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Totals
Change from base Build 6428254063:	0.0%
Covered Lines:	2956
Relevant Lines:	3058

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💛 - Coveralls