Benchmark before:
```
goos: darwin
goarch: arm64
pkg: github.com/go-logr/logr/benchmark
BenchmarkDiscardLogInfoOneArg-10 58466427 21.36 ns/op 32 B/op 1 allocs/op
BenchmarkDiscardLogInfoSeveralArgs-10 24470527 49.62 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogInfoWithValues-10 21938677 47.06 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogV0Info-10 24796845 47.45 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogV9Info-10 26069977 49.02 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogError-10 23731986 47.94 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardWithValues-10 50995291 22.28 ns/op 64 B/op 1 allocs/op
BenchmarkDiscardWithName-10 1000000000 0.4716 ns/op 0 B/op 0 allocs/op
BenchmarkFuncrLogInfoOneArg-10 2684097 433.9 ns/op 1152 B/op 7 allocs/op
BenchmarkFuncrJSONLogInfoOneArg-10 2413437 480.4 ns/op 1168 B/op 8 allocs/op
BenchmarkFuncrLogInfoSeveralArgs-10 1261210 953.2 ns/op 1448 B/op 17 allocs/op
BenchmarkFuncrJSONLogInfoSeveralArgs-10 1000000 1003 ns/op 1448 B/op 18 allocs/op
BenchmarkFuncrLogInfoWithValues-10 1247379 1027 ns/op 1480 B/op 17 allocs/op
BenchmarkFuncrJSONLogInfoWithValues-10 1000000 1109 ns/op 1496 B/op 18 allocs/op
BenchmarkFuncrLogV0Info-10 1000000 1043 ns/op 1448 B/op 17 allocs/op
BenchmarkFuncrJSONLogV0Info-10 1000000 1100 ns/op 1448 B/op 18 allocs/op
BenchmarkFuncrLogV9Info-10 24429925 48.76 ns/op 176 B/op 2 allocs/op
BenchmarkFuncrJSONLogV9Info-10 24799878 53.82 ns/op 176 B/op 2 allocs/op
BenchmarkFuncrLogError-10 1000000 1010 ns/op 1480 B/op 19 allocs/op
BenchmarkFuncrJSONLogError-10 1000000 1124 ns/op 1496 B/op 20 allocs/op
BenchmarkFuncrWithValues-10 4088592 296.0 ns/op 264 B/op 8 allocs/op
BenchmarkFuncrWithName-10 34046577 32.95 ns/op 96 B/op 1 allocs/op
BenchmarkFuncrWithCallDepth-10 34601535 32.77 ns/op 96 B/op 1 allocs/op
BenchmarkFuncrJSONLogInfoStringerValue-10 2062101 562.0 ns/op 1200 B/op 10 allocs/op
BenchmarkFuncrJSONLogInfoErrorValue-10 2278339 529.9 ns/op 1200 B/op 10 allocs/op
BenchmarkFuncrJSONLogInfoMarshalerValue-10 2108040 533.6 ns/op 1200 B/op 10 allocs/op
```
Benchmark after:
```
goos: darwin
goarch: arm64
pkg: github.com/go-logr/logr/benchmark
BenchmarkDiscardLogInfoOneArg-10 59617632 20.01 ns/op 32 B/op 1 allocs/op
BenchmarkDiscardLogInfoSeveralArgs-10 26171325 47.09 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogInfoWithValues-10 25975759 45.76 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogV0Info-10 25654935 46.02 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogV9Info-10 25722668 46.00 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardLogError-10 25181846 45.97 ns/op 176 B/op 2 allocs/op
BenchmarkDiscardWithValues-10 53340940 21.92 ns/op 64 B/op 1 allocs/op
BenchmarkDiscardWithName-10 1000000000 0.4757 ns/op 0 B/op 0 allocs/op
BenchmarkFuncrLogInfoOneArg-10 2887884 416.5 ns/op 1152 B/op 7 allocs/op
BenchmarkFuncrJSONLogInfoOneArg-10 2556847 474.0 ns/op 1168 B/op 8 allocs/op
BenchmarkFuncrLogInfoSeveralArgs-10 1280838 957.4 ns/op 1448 B/op 17 allocs/op
BenchmarkFuncrJSONLogInfoSeveralArgs-10 1000000 1003 ns/op 1448 B/op 18 allocs/op
BenchmarkFuncrLogInfoWithValues-10 1241484 967.6 ns/op 1480 B/op 17 allocs/op
BenchmarkFuncrJSONLogInfoWithValues-10 1000000 1021 ns/op 1496 B/op 18 allocs/op
BenchmarkFuncrLogV0Info-10 1202900 925.3 ns/op 1448 B/op 17 allocs/op
BenchmarkFuncrJSONLogV0Info-10 1000000 1021 ns/op 1448 B/op 18 allocs/op
BenchmarkFuncrLogV9Info-10 24424683 48.45 ns/op 176 B/op 2 allocs/op
BenchmarkFuncrJSONLogV9Info-10 24633237 48.23 ns/op 176 B/op 2 allocs/op
BenchmarkFuncrLogError-10 1000000 1066 ns/op 1480 B/op 19 allocs/op
BenchmarkFuncrJSONLogError-10 1072557 1093 ns/op 1496 B/op 20 allocs/op
BenchmarkFuncrWithValues-10 4094355 292.9 ns/op 264 B/op 8 allocs/op
BenchmarkFuncrWithName-10 36661557 33.96 ns/op 96 B/op 1 allocs/op
BenchmarkFuncrWithCallDepth-10 32008606 32.65 ns/op 96 B/op 1 allocs/op
BenchmarkFuncrJSONLogInfoStringerValue-10 2329975 517.9 ns/op 1200 B/op 10 allocs/op
BenchmarkFuncrJSONLogInfoErrorValue-10 2200308 522.0 ns/op 1200 B/op 10 allocs/op
BenchmarkFuncrJSONLogInfoMarshalerValue-10 2242267 536.0 ns/op 1200 B/op 10 allocs/op
```
The performance is similar. Only a nil pointer check is added in the main path.
Signed-off-by: spacewander <spacewanderlzx@gmail.com>