[^] should be valid

```c++
#include <ctre.hpp>

bool test(const char * str)
{
    return ctre::match<"[^]">(str);
}
```

Expected: Should match exactly 1 byte that is not a member of the empty set, aka check if the string's length is 1.

Actual: 965 lines of compiler output, ending with `invalid use of incomplete type 'struct ctre::problem_at_position<2>'`.

https://godbolt.org/z/cvPfsaqcM