Fix batcher panic on invalid configuration

[mdehoog]

Description

The batcher panics if an invalid configuration is passed. This PR fixes the nil access so that the actual configuration error is printed, rather than a panic.

Tests

Added small regression test.

Additional context

It can be easily repro'd using (without this PR's changes):

make op-batcher && bin/op-batcher --l1-eth-rpc https://example.com --l2-eth-rpc https://example.com --rollup-rpc https://example.com

INFO [10-31|12:39:25.132] Initializing Batch Submitter
INFO [10-31|12:39:25.606] Stopping batcher
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x101623288]

goroutine 1 [running]:
github.com/ethereum-optimism/optimism/op-batcher/batcher.(*BatchSubmitter).StopBatchSubmitting(0x0, {0x101ddca60, 0x14000180690})
	/Users/michaeldehoog/src/optimism/op-batcher/batcher/driver.go:114 +0x38
github.com/ethereum-optimism/optimism/op-batcher/batcher.(*BatchSubmitter).StopBatchSubmittingIfRunning(0x1?, {0x101ddca60?, 0x14000180690?})
	/Users/michaeldehoog/src/optimism/op-batcher/batcher/driver.go:105 +0x24
github.com/ethereum-optimism/optimism/op-batcher/batcher.(*BatcherService).Stop(0x140002f2800, {0x101ddca60, 0x14000180690})
	/Users/michaeldehoog/src/optimism/op-batcher/batcher/service.go:292 +0x88
github.com/ethereum-optimism/optimism/op-batcher/batcher.BatcherServiceFromCLIConfig({0x101ddca60, 0x14000180690}, {0x101b8a6d8, 0x6}, 0x2c?, {0x101de2bf8, 0x14000494540})
	/Users/michaeldehoog/src/optimism/op-batcher/batcher/service.go:74 +0x84
main.main.Main.func1(0x140003e4340, 0x1025a1a00?)
	/Users/michaeldehoog/src/optimism/op-batcher/batcher/batch_submitter.go:32 +0x15c
main.main.LifecycleCmd.lifecycleCmd.func3(0x140003e4340)
	/Users/michaeldehoog/src/optimism/op-service/cliapp/lifecycle.go:56 +0x110
github.com/urfave/cli/v2.(*Command).Run(0x1400011cdc0, 0x140003e4340, {0x140001d6000, 0x7, 0x7})
	/Users/michaeldehoog/go/pkg/mod/github.com/urfave/cli/v2@v2.25.7/command.go:274 +0x730
github.com/urfave/cli/v2.(*App).RunContext(0x140000125a0, {0x101ddc788?, 0x1025a1a00}, {0x140001d6000, 0x7, 0x7})
	/Users/michaeldehoog/go/pkg/mod/github.com/urfave/cli/v2@v2.25.7/app.go:332 +0x518
github.com/urfave/cli/v2.(*App).Run(...)
	/Users/michaeldehoog/go/pkg/mod/github.com/urfave/cli/v2@v2.25.7/app.go:309
main.main()
	/Users/michaeldehoog/src/optimism/op-batcher/cmd/main.go:41 +0x4fc