fix: NPE on empty pool configuration

[rustatian]

Reason for This PR

• NPE if the user doesn't use an activities in the configuration

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x18 pc=0x1b84eea]

goroutine 1 [running]:
github.com/roadrunner-server/sdk/v2/pool.(*Config).InitDefaults(...)
        github.com/roadrunner-server/sdk/v2@v2.19.0/pool/config.go:39
github.com/roadrunner-server/sdk/v2/pool.NewStaticPool({0x2caf940?, 0xc00004a0e8?}, 0xc000934c60?, {0x2ca8050?, 0xc00062cb68?}, {0x262a2a0?, 0x0?}, 0x110?)
        github.com/roadrunner-server/sdk/v2@v2.19.0/pool/static_pool.go:58 +0x6a
github.com/roadrunner-server/server/v2.(*Plugin).NewWorkerPool(0xc000662f00, {0x2caf940, 0xc00004a0e8}, {0x262a2a0, 0x0}, 0x8?, 0x30fcfdd?)
        github.com/roadrunner-server/server/v2@v2.16.4/plugin.go:246 +0x125
github.com/temporalio/roadrunner-temporal.(*Plugin).initPool(0xc000856140)
        github.com/temporalio/roadrunner-temporal@v1.7.0/plugin.go:489 +0x126
github.com/temporalio/roadrunner-temporal.(*Plugin).Serve(0xc000856140)
        github.com/temporalio/roadrunner-temporal@v1.7.0/plugin.go:275 +0x14b0
reflect.Value.call({0xc000b86540?, 0xc0002096d0?, 0x1f6d09e0a28?}, {0x2966d97, 0x4}, {0xc0007df148, 0x1, 0x2ccfef0?})
        reflect/value.go:584 +0x8c5
reflect.Value.Call({0xc000b86540?, 0xc0002096d0?, 0x0?}, {0xc0007df148?, 0x2ccfef0?, 0xc000b86540?})
        reflect/value.go:368 +0xbc
github.com/roadrunner-server/endure/pkg/container.(*Endure).callServeFn(0xc0006247e0, 0xc000266f20, {0xc0007df148, 0x1, 0x1})
        github.com/roadrunner-server/endure@v1.4.5/pkg/container/serve.go:18 +0x24b
github.com/roadrunner-server/endure/pkg/container.(*Endure).serveInternal(0xc0006247e0, 0xc00022e420)
        github.com/roadrunner-server/endure@v1.4.5/pkg/container/serve.go:50 +0x1c7
github.com/roadrunner-server/endure/pkg/container.(*Endure).Start(0xc0006247e0)
        github.com/roadrunner-server/endure@v1.4.5/pkg/container/endure.go:445 +0x85
reflect.Value.call({0xc000106840?, 0xc00062c3b8?, 0x1f6d09e0a28?}, {0x2966d97, 0x4}, {0xc00022e9a8, 0x1, 0xd5f3d6?})
        reflect/value.go:584 +0x8c5
reflect.Value.Call({0xc000106840?, 0xc00062c3b8?, 0xc000000001?}, {0xc00022e9a8?, 0xc000008e10?, 0x0?})
        reflect/value.go:368 +0xbc
github.com/roadrunner-server/endure/pkg/fsm.(*FSMImpl).Transition(0xc000008c48, 0x1, {0xc00023d520, 0x1, 0x1?})
github.com/roadrunner-server/endure/pkg/container.(*Endure).Serve(0xc0006247e0)
        github.com/roadrunner-server/endure@v1.4.5/pkg/container/endure.go:327 +0xd9
github.com/roadrunner-server/roadrunner/v2/internal/cli/serve.NewCommand.func1(0xc000827180, {0x2967367, 0x0, 0x0})
        github.com/roadrunner-server/roadrunner/v2/internal/cli/serve/command.go:66 +0x445
github.com/spf13/cobra.(*Command).execute(0xc000827180, {0x3d89978, 0x0, 0x0})
        github.com/spf13/cobra@v1.5.0/command.go:872 +0x694
github.com/spf13/cobra.(*Command).ExecuteC(0xc000826a00)
        github.com/spf13/cobra@v1.5.0/command.go:990 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/cobra@v1.5.0/command.go:918
        github.com/roadrunner-server/roadrunner/v2/cmd/rr/main.go:21 +0x45
        github.com/roadrunner-server/roadrunner/v2/cmd/rr/main.go:15 +0x19

Description of Changes

• Properly initialize the pool of workers configuration.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.