fix division by zero panic when basefee update fraction is 0

[pk910]

URL: /slot/64
Time: 2025-10-15 16:51:06.430000847 +0200 CEST m=+21.586654647
Version: git-

Error:
page call 4 panic: division by zero

Stack Trace:
goroutine 343 [running]:
runtime/debug.Stack()
	/home/pk910/libs/src/golang/1.25.0/go/src/runtime/debug/stack.go:26 +0x5e
github.com/ethpandaops/dora/services.(*FrontendCacheService).processPageCall.func1.1()
	/home/pk910/github/ethpandaops/dora/services/frontendcache.go:133 +0xd4
panic({0x239e800?, 0x4bee8a0?})
	/home/pk910/libs/src/golang/1.25.0/go/src/runtime/panic.go:783 +0x132
math/big.nat.div({0x0?, 0x400000043861b?, 0x7fa7146c2c60?}, 0x7fa7656a9a78?, {0x0?, 0xc000500808?, 0xc00259c780?}, {0x0, 0x0, 0x0}, ...)
	/home/pk910/libs/src/golang/1.25.0/go/src/math/big/natdiv.go:520 +0x4b5
math/big.(*Int).QuoRem(0xc00264c998, 0xc00264c978, 0xc00264c958, 0xc00264c810)
	/home/pk910/libs/src/golang/1.25.0/go/src/math/big/int.go:300 +0x8c
math/big.(*Int).Div(0xc00264c998, 0x0?, 0xc00264c8b0?)
	/home/pk910/libs/src/golang/1.25.0/go/src/math/big/int.go:311 +0x38
github.com/ethpandaops/dora/clients/execution.(*ChainState).CalcBaseFeePerBlobGas(0xc0001dab40?, 0x0, 0x0)
	/home/pk910/github/ethpandaops/dora/clients/execution/chainstate.go:335 +0x165
github.com/ethpandaops/dora/handlers.getSlotPageBlockData(0xc0025e4680, 0xc00213a120)
	/home/pk910/github/ethpandaops/dora/handlers/slot.go:738 +0x2c10
github.com/ethpandaops/dora/handlers.buildSlotPageData({0x4c11a10, 0xc0025eb3b0}, 0x40, {0x6942960, 0x0, 0x4619ce?})
	/home/pk910/github/ethpandaops/dora/handlers/slot.go:278 +0x550
github.com/ethpandaops/dora/handlers.getSlotPageData.func1(0xc0025e87e0)
	/home/pk910/github/ethpandaops/dora/handlers/slot.go:179 +0x32
github.com/ethpandaops/dora/services.(*FrontendCacheService).processPageCall.func1(0xc002c95400?)
	/home/pk910/github/ethpandaops/dora/services/frontendcache.go:150 +0x1d0
created by github.com/ethpandaops/dora/services.(*FrontendCacheService).processPageCall in goroutine 103
	/home/pk910/github/ethpandaops/dora/services/frontendcache.go:127 +0x348