Fix Comparators.nullsLow and Comporators.nullsHigh behavior

Commit 33454a4007 introduced a regression in Comparators.nullsLow() and
Comporators.nullsHigh().

This commit updates the code so that nullsLow() sorts null values lower
than non-null values and nullsHigh sorts null values higher than
non-null values.

See gh-25478
Closes gh-31808

spring-core/src/test/java/org/springframework/util/comparator/ComparatorsTest.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.util.comparator;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link Comparators}
+ */
+class ComparatorsTest {
+
+	@Test
+	void shouldCompareWithNullsLow() {
+		assertThat(Comparators.nullsLow().compare(null, "boo")).isNegative();
+		assertThat(Comparators.nullsLow().compare(null, null)).isZero();
+	}
+
+	@Test
+	void shouldCompareWithNullsLowAndComparator() {
+		assertThat(Comparators.nullsLow(String::compareTo).compare(null, "boo")).isNegative();
+		assertThat(Comparators.nullsLow(String::compareTo).compare(null, null)).isZero();
+	}
+
+	@Test
+	void shouldCompareWithNullsHigh() {
+		assertThat(Comparators.nullsHigh().compare(null, "boo")).isPositive();
+		assertThat(Comparators.nullsHigh().compare(null, null)).isZero();
+	}
+
+	@Test
+	void shouldCompareWithNullsHighAndComparator() {
+		assertThat(Comparators.nullsHigh(String::compareTo).compare(null, "boo")).isPositive();
+		assertThat(Comparators.nullsHigh(String::compareTo).compare(null, null)).isZero();
+	}
+}