[R] Don't throw `stratified=FALSE` warning when nothing changes #11353

Job	Run time
Build CI containers (xgb-ci.manylinux2014_x86_64)	34s
Build libxgboost4j.dylib for MacOS (Apple Silicon)	1m 31s
Build CI containers (xgb-ci.jvm)	38s
Build libxgboost4j.dylib for MacOS (Intel)	5m 37s
Build CI containers (xgb-ci.jvm_gpu_build)	1m 24s
Build CI containers (xgb-ci.manylinux2014_aarch64)	27s
Build and test JVM packages (windows-latest)	8m 42s
Build and test JVM packages (macos-13)	9m 23s
Build libxgboost4j.so targeting glibc 2.17 (arch aarch64, runner linux-arm64-cpu)	1m 41s
Build libxgboost4j.so targeting glibc 2.17 (arch x86_64, runner linux-amd64-cpu)	1m 57s
Build libxgboost4j.so with CUDA	5m 36s
Build and test JVM packages (Linux, Scala 2.12)	9m 37s
Build and test JVM packages (Linux, Scala 2.13)	5m 2s
Build docs for JVM packages	4m 40s
Test JVM packages with CUDA (Scala 2.12)	5m 59s
Test JVM packages with CUDA (Scala 2.13)	6m 3s
Deploy JVM packages to S3 (cpu)	39s
Deploy JVM packages to S3 (cpu)	38s
Deploy JVM packages to S3 (gpu)	1m 27s
Deploy JVM packages to S3 (gpu)	1m 24s
	1h 12m 59s

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