Change gtGetThisArg not to return nullptr. (#44398)

Change gtGetThisArg not to return nullptr. (#44398)

* Don't wrap string literal const as nop for CoreRT.

It was probably an old workaround for another Jit bug, it is most likely fixed by now.

* Change `gtGetThisArg` not to return `nullptr`.

There was only 1 case where a null return was tolerated - for a tail call in `optAssertionGen` marked as virtual.
However, a transformed tail call is never a virtual, fix `fgMorphTailCallViaJitHelper` to unset virtual flag.

* Keep x86 tail call via jit helper as a virtual stub call.