Polish "Use `singleOrEmpty()` instead of `buffer()`"

See gh-31282

spring-r2dbc/src/main/java/org/springframework/r2dbc/core/DefaultFetchSpec.java

@@ -61,7 +61,7 @@ class DefaultFetchSpec<T> implements FetchSpec<T> {
 	public Mono<T> one() {
 		return all().singleOrEmpty()
 			.onErrorMap(IndexOutOfBoundsException.class, ex -> {
-				String message = String.format("Query [%s] returned non unique result.", resultFunction.getSql());
+				String message = String.format("Query [%s] returned non unique result.", this.resultFunction.getSql());
 				return new IncorrectResultSizeDataAccessException(message, 1);
 			});
 	}