Fix: panic when config.ecs.launch_type is nil

[fujiwara]

Fix panic raised when launch_type is nil (not defined in yaml).

2024-03-29T05:47:33.811Z [info] [ecs.go:190] launching task subdomain:com taskdef:dev-app
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xcc36f5]
goroutine 161 [running]:
github.com/acidlemon/mirage-ecs/v2.(*ECS).launchTask(0xc0002e71a0, {0xfd2290, 0xc00012cc40}, {0xc0003b006a, 0x3}, {0xc0003b0084, 0x7}, 0xc000462240?)
	/stash/src/github.com/acidlemon/mirage-ecs/ecs.go:220 +0x735
github.com/acidlemon/mirage-ecs/v2.(*ECS).Launch.func1()
	/stash/src/github.com/acidlemon/mirage-ecs/ecs.go:259 +0x33
golang.org/x/sync/errgroup.(*Group).Go.func1()
	/stash/pkg/mod/golang.org/x/sync@v0.3.0/errgroup/errgroup.go:75 +0x56
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 145
	/stash/pkg/mod/golang.org/x/sync@v0.3.0/errgroup/errgroup.go:72 +0x96