Don't store nullptrs in mlir::FuncOp::getAll*Attrs' result

These methods for results and arguments would create an ArrayRef full
of nullptrs when there were no argument attributes. This is problematic
because this result could not be passed to the FuncOp::build creator
without causing a segfault. Now the list will have empty attributes.

Differential Revision: https://reviews.llvm.org/D114358