Allow condacts without return values

[chsticksel]

A condact without return values does not have default values, parsing

condact(clock, N(a,b)) is syntactically not allowed at the moment

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• Bitbucket: https://bitbucket.org/kind2-mc/kind2/issue/19
• Originally reported by: chsticksel
• Originally created at: 2014-11-26T23:09:50.133