Add null check to string_view

[Fernthedev]

There's a chance for undefined behaviour in il2cpp string allocations when the given string is null. This attempts to make it more explicit what should occur.

[sc2ad]

Approved after feedback is addressed

[RedBrumbler]

Before merging things, I think it's warranted to think about whether a null string view should return a non-null StringW, as in my view this is weird behaviour. I think a null string view should return a null StringW.

For a final decision I would like to see the use case that brought this issue to light, and decide from there

[RedBrumbler]

as it turns out, the std::string and std::string_view behaviour for a default constructed (null) string view is to produce a valid empty string

[sc2ad]

As shown by your first log line (specifically with a.data()): we see a null string so it's clear there is AN issue with the existing implementation. Converting a string view with a null pointer to a stringw is fundamentally odd, though as you mentioned in C++, an empty (but valid) std::string is made. So, I'm going to merge this impl and we can revise further in the future.

[RedBrumbler]

yes we do see a null string there, but my point was that the standard library turns that into a valid empty string, I also still think it's weird behaviour but if it's what the standard library does I think we should do the same (as is done by merging this ofc)