Improve health contributor null support

Update `NamedContributorsMapAdapter` to check for `null` keys or values
during construction. Also update `HealthEndpointSupport` to allow
null component entries.

See gh-18687

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/health/NamedContributorsMapAdapterTests.java

@@ -48,6 +48,22 @@ class NamedContributorsMapAdapterTests {
 				.withMessage("ValueAdapter must not be null");
 	}
 
+	@Test
+	void createWhenMapContainsNullValueThrowsException() {
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> new TestNamedContributorsMapAdapter<>(Collections.singletonMap("test", null),
+						Function.identity()))
+				.withMessage("Map must not contain null values");
+	}
+
+	@Test
+	void createWhenMapContainsNullKeyThrowsException() {
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> new TestNamedContributorsMapAdapter<>(Collections.singletonMap(null, "test"),
+						Function.identity()))
+				.withMessage("Map must not contain null keys");
+	}
+
 	@Test
 	void iterateReturnsAdaptedEntries() {
 		TestNamedContributorsMapAdapter<String> adapter = createAdapter();