Add unit tests to address new string routine `string_is_empty`

#160 introduced a new string function, we should have a couple of unit tests to ensure that it works in all cases.

I'm sure it does, but the more string unit tests the better, because at least they are passing.

>**Fixes #166 - Implemented unit tests for webvtt_string_is_empty**
>
>Implemented tests for `webvtt_string_is_empty` and `String::isEmpty`,
>which tests both that we return true when the length is 0, and false
>when the length is greater than 0.
>
>I found during these tests that we are not initializing strings
>correctly in the C++ bindings, which can easily lead to segfaults.
>Perhaps the `webvtt_init_string` and other routines should be changed to
>behave differently, but currently the string to be initialized must be
>0 to begin with, otherwise it will be dereferenced which can cause
>problems.