Literals should be able to start with NULL character

According to documentation on `hs_compile_lit`, "the special terminating character \0 should be allowed to appear in expression, and not treated as a terminator for a string". Recent addition of `strcmp` check for empty expressions invalidates this statement, specifically meaning they can no longer start with a '\0'.

This is related to issue #386, though unlike that issue, where an additional NULL terminator can be added to expressions, there is no obvious workaround for this for any expression required to start with a NULL.