Allow sign message nil payload

[g-k]

fixes #12

[coveralls]

Pull Request Test Coverage Report for Build 187

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 91.667%

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Totals
Change from base Build 184:	0.06%
Covered Lines:	484
Relevant Lines:	528

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💛 - Coveralls