FmIndex breaks if text is empty string

[faassen]

FmIndex currently breaks with an error if the text supplied is the empty string (an underflow error).

I also tried with a string just with a \0 in it but had a different error:

sampling level L must satisfy 2^L < text_len (L = 2, text_len = 4)

[faassen]

The failing test is this:

    #[test]
    fn test_empty() {
        let text = "".to_string().into_bytes();
        let fm_index = FMIndex::new(
            text,
            IdConverter::new(256),
            SuffixOrderSampler::new().level(2),
        );
        assert_eq!(fm_index.search_backward("").count(), 0);
    }

I notice that if I set level to 0, it does work. As I asked in #23, I don't really understand level or how to set it to a sane number.

[faassen]

Oh, wait, it doesn't work if I set it to empty with level(0), as I get the underflow error described originally.

[faassen]

For my use case an empty string is okay to ignore, as I always end them with \0 anyway. But I think we should give better feedback if it's used that way.