Partial fix for bug #61 "empty character class [] matches empty string".

Partial fix for bug #61 "empty character class [] matches empty string".

Given the following code:
    /*!re2c
        [] {}
    */

    /*!re2c
        [^\x00-\xFF] {}
    */

    /*!re2c
        [\x00-\xFF]\[\x00-\xFF] {}
    */
re2c versions <=0.13.6 and >=0.13.7 behaved differently.
0.13.6 consistently considered that empty range should match empty string.
Since 0.13.7 empty positive range [] and empty difference (e.g. [a-z][a-z])
still match empty string, but empty negative range (e.g. [^\x00-\xFF])
matches nothing (always fails). The faulty commit is
28ee7c95bca46ad3cdb965741c5c29e21c50df14
"Added UTF-8 encoding support and tests for it."

This commit brings back consistent behaviour of 0.13.6: empty range,
however it was constructed, always matches empty string. Whether this
behaviour is sane or not is another question.