Add `AddNull` argument to `CreateGlobalString`. (#93036)

There's currently no way to control whether a null terminator should be
appended to the string created in `CreateGlobalString` /
`CreateGlobalStringPtr`, since the methods don't expose an additional
argument.
This change adds an additional argument to the methods that has the same
default value, `true`, as in `ConstantDataArray::getString`, and passes
it down to this internal method.