Handle empty zap.Field{} to avoid panics

[marclop]

This commit fixes a panic that occurs when a zap.Field{} is passed to the logger. This is done by wrapping the passed TopicLogFieldFunc in a function that checks if the field is empty and returns zap.Skip() if it is.

[marclop]

With commit 10c4c75 the logger's encoder panics due to using an empty zap.Field{} (https://github.com/elastic/apm-queue/actions/runs/11174157829/job/31063376953?pr=581):

2024-10-04T04:47:44.8534949Z go test -race  -count=10 -timeout=60s ./...
2024-10-04T04:47:51.7799855Z ?   	github.com/elastic/apm-queue/v2	[no test files]
2024-10-04T04:48:11.7757225Z ?   	github.com/elastic/apm-queue/v2/cmd/queuebench	[no test files]
2024-10-04T04:48:13.5729793Z ?   	github.com/elastic/apm-queue/v2/metrictest	[no test files]
2024-10-04T04:48:14.9044084Z panic: unknown field type: { 0 0  <nil>}
2024-10-04T04:48:14.9044758Z 
2024-10-04T04:48:14.9045015Z goroutine 11807 [running]:
2024-10-04T04:48:14.9049320Z go.uber.org/zap/zapcore.Field.AddTo({{0x0, 0x0}, 0x0, 0x0, {0x0, 0x0}, {0x0, 0x0}}, {0x7ff5d782bf38, 0xc000a3e640})
2024-10-04T04:48:14.9053620Z 	/home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/zapcore/field.go:180 +0xe99
2024-10-04T04:48:14.9054888Z go.uber.org/zap/zapcore.addFields(...)
2024-10-04T04:48:14.9056101Z 	/home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/zapcore/field.go:210
2024-10-04T04:48:14.9057783Z go.uber.org/zap/zapcore.(*ioCore).With(0xc000792b70, {0xc000a3e600, 0x1, 0x44aa85?})
2024-10-04T04:48:14.9059133Z 	/home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/zapcore/core.go:83 +0x329
2024-10-04T04:48:14.9059878Z go.uber.org/zap.(*Logger).With(0xc000a70480, {0xc000a3e600, 0x1, 0x1})
2024-10-04T04:48:14.9060700Z 	/home/runner/go/pkg/mod/go.uber.org/zap@v1.27.0/logger.go:185 +0xde
2024-10-04T04:48:14.9061934Z github.com/elastic/apm-queue/v2/kafka.(*consumer).assigned(0xc000acef50, {0xec6925?, 0x135ff00?}, 0xc000554808, 0xc000792b40)
2024-10-04T04:48:14.9063046Z 	/home/runner/work/apm-queue/apm-queue/kafka/consumer.go:422 +0x616
2024-10-04T04:48:14.9064003Z github.com/twmb/franz-go/pkg/kgo.(*consumer).initGroup.func2({0x1677488, 0xc000ae9130}, 0xc000554808, 0xc0007929c0)
2024-10-04T04:48:14.9065121Z 	/home/runner/go/pkg/mod/github.com/twmb/franz-go@v1.17.0/pkg/kgo/consumer_group.go:343 +0x56f
2024-10-04T04:48:14.9066099Z github.com/twmb/franz-go/pkg/kgo.(*assignRevokeSession).assign.func1()
2024-10-04T04:48:14.9067669Z 	/home/runner/go/pkg/mod/github.com/twmb/franz-go@v1.17.0/pkg/kgo/consumer_group.go:795 +0x2df
2024-10-04T04:48:14.9068957Z created by github.com/twmb/franz-go/pkg/kgo.(*assignRevokeSession).assign in goroutine 11720
2024-10-04T04:48:14.9070364Z 	/home/runner/go/pkg/mod/github.com/twmb/franz-go@v1.17.0/pkg/kgo/consumer_group.go:784 +0x11d
2024-10-04T04:48:14.9071294Z FAIL	github.com/elastic/apm-queue/v2/kafka	1.325s
2024-10-04T04:48:14.9072021Z ok  	github.com/elastic/apm-queue/v2/queuecontext	1.026s
2024-10-04T04:48:14.9072847Z ok  	github.com/elastic/apm-queue/v2/systemtest	1.017s