A valid unsubscribe package must contain at least one topic filter

[gausby]

Updated the unsubscribe package test generator to not generate empty subscription packages as well.

If applied this should fix #69

[coveralls]

Pull Request Test Coverage Report for Build 496

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 87.287%

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Files with Coverage Reduction	New Missed Lines	%
lib/tortoise/connection/receiver.ex	2	84.38%

Totals
Change from base Build 482:	-0.04%
Covered Lines:	563
Relevant Lines:	645

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💛 - Coveralls