Bug: apply_sparse_delta crashes with StopIteration when base_state is empty and target_dtype is None

[Grizouforever]

Description

apply_sparse_delta in grail/infrastructure/delta_checkpoint.py (line 137) crashes with an unhandled StopIteration exception when base_state is an empty dict and target_dtype is None.

if target_dtype is None:
    target_dtype = next(iter(base_state.values())).dtype  # StopIteration if base_state == {}

This is called from checkpoint_consumer.py at lines 439-444 and 1315-1319 with target_dtype=None, making those call sites vulnerable to the crash.

Steps to Reproduce

from grail.infrastructure.delta_checkpoint import apply_sparse_delta

apply_sparse_delta({}, {}, {}, target_dtype=None)
# StopIteration raised, not caught by caller

Expected Behavior

A clear ValueError with a descriptive message: "Cannot infer target_dtype: base_state is empty".

Actual Behavior

StopIteration propagates up through the call stack, crashing the caller.