message: Fix an actually invalid test
  C++ overload resolution/list initialization rules are hard.
diff: @@ -1120,7 +1120,7 @@ TEST_CASE("constructors")
  SECTION("constructor with implicit types (array)")
  {
  - json j {std::move(source)};
  + json j {std::move(source), {}};
  CHECK(&j[0][0] == source_addr);
  }
  @@ -1132,7 +1132,7 @@ TEST_CASE("constructors")
  SECTION("assignment with implicit types (array)")
  {
  - json j = {std::move(source)};
  + json j = {std::move(source), {}};
  CHECK(&j[0][0] == source_addr);
  }
  