block: guard zero-value smelter experience

[HashimTheArab]

Summary

Guard smelter.Experience against nil receivers so zero-value furnace, blast furnace, and smoker block values report no stored XP instead of panicking when their BreakInfo is computed.

Zero-value smelter blocks can appear as stateless block registry or palette values. Initializing those prototypes with newSmelter would give shared mutable inventory state to the registered block value, so treating missing smelter state as zero stored XP keeps the stateful data on decoded block entities.

Validation

• Reproduced the upstream panic locally with a temporary test against block.Furnace{}, block.BlastFurnace{}, and block.Smoker{} calling BreakInfo().XPDrops.
• Verified the same temporary test passes after the fix, then removed it from the branch as requested.
• go test ./server/... -count=1