InvokeStatic: pass (nullptr, 0) when there are no args

Previously, the behavior depended on how the STL implemented a
std::array of zero length. libc++'s behavior changed in D80821 from a
non-null data() to a null data().

In InvokeInstance, vregs will always have at least the receiver, so
data() will never be null.

Bug: b/293402944
Test: treehugger
Change-Id: Ibd2c772baa14993b2fbdd2abdb6b58621dca0c5c