Process epoch error to use correct state version

[terencechain]

h/t Seamonkey for pointing out a regression from #14001.

If the ProcessEpoch function returns an error, the state is nil by default. We can't use that to get the state version, or it will panic as seen below:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x468 pc=0x37a1d60]

goroutine 607921584 [running]:
github.com/prysmaticlabs/prysm/v5/beacon-chain/core/transition.ProcessSlots({0x1b5eb88?, 0xc10910b1f0?}, {0x1b8abe8, 0xc10de18700}, 0x1b16a0)
        beacon-chain/core/transition/transition.go:263 +0xba0
github.com/prysmaticlabs/prysm/v5/beacon-chain/core/transition.UpdateNextSlotCache({0x1b5eb88, 0xc10910b1f0}, {0xc07f7ffea0, 0x20, 0x1ed738942d8c?}, {0x1b8abe8?, 0xc05b936a80?})

This PR fixes the caller return pattern since state is mutated, we don't need to return it

[prestonvanloon]

Can we drop the returned state? ProcessEpoch mutates the state argument, there is no benefit to conditionally returnign the state here. Refactor the method signature to only return an error.

Suggested change

	state, err = altair.ProcessEpoch(ctx, state)
	err = altair.ProcessEpoch(ctx, state)

[james-prysm]

maybe we should do it outside , but it's also possible to trigger the nil pointer issue in the else statement maybe. It was my idea to add this state version but seems to be causing other issues.