Fix null pointer dereference in xla::cpu::CustomCallOpLowering::rewriteTypedCustomCall()
Leaving to XLA team to review @ezhulenev, can you take a look? @frgossen, can you review this fix, please?
The bug was found by Svace static analyzer:

1. op.getBackendConfig() can be null
2. dict will be nullptr
3. dict.begin() dereferences a null pointer

Closes #60223

cc @mihaimaruseac