Resolve #8: Fix typestring_is("") case

[nathanielhourt]

I find that typestring_is("") gives back a typestring<'\0'> unless I add these overloads to give back a typestring<> instead. The typoke overload fixes it in the majority of cases, whereas the typeek overload is necessary in the USE_TYPESTRING == 0 case.

[nathanielhourt]

Resolves issue #8