[WebAssembly] Handle empty cleanuppads when adding catch_all

In `LateEHPrepare::addCatchAlls`, the current code tries to get the
iterator's debug info even when it is `MachineBasicBlock::end()`. This
fixes the bug by adding empty debug info instead in that case.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D97679