Coq: allow use of the concurrency interface without an exception type

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Test Results

    9 files  ±0     20 suites  ±0   0s :stopwatch: ±0s   649 tests ±0    649 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  2 076 runs  ±0  2 075 :white_check_mark: ±0  1 :zzz: ±0  0 :x: ±0 

Results for commit e1407f5c. ± Comparison against base commit ff355c85.