luatest prints nil when message in assertion is empty #171
Description
When testcase contains a skip_if without an optional message luatest prints nil after test status:
sergeyb@pony:~/sources/MRG/expirationd$ ./.rocks/bin/luatest -v test/unit/start_key_test.lua
Started on Wed Sep 22 17:51:26 2021
start_key.engine:"memtx".index_type:"BITSET".test_passing ... (0.010s) skip
nil
start_key.engine:"memtx".index_type:"BITSET".test_tree_index ... (0.009s) skip
...eyb/sources/MRG/expirationd/test/unit/start_key_test.lua:73: Unsupported index type
start_key.engine:"memtx".index_type:"BITSET".test_hash_index ... (0.010s) skip
...eyb/sources/MRG/expirationd/test/unit/start_key_test.lua:130: Unsupported index type
start_key.engine:"memtx".index_type:"HASH".test_passing ... (0.011s) skip
nil
start_key.engine:"memtx".index_type:"HASH".test_tree_index ... (0.012s) skip
luatest 0.5.5