Polish "Assert that sources does not contain null elements"

See gh-30878

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/Binder.java

@@ -182,7 +182,9 @@ public class Binder {
 			List<ConversionService> conversionServices, Consumer<PropertyEditorRegistry> propertyEditorInitializer,
 			BindHandler defaultBindHandler, BindConstructorProvider constructorProvider) {
 		Assert.notNull(sources, "Sources must not be null");
-		sources.forEach((source) -> Assert.notNull(source, "Sources cannot contain null values"));
+		for (ConfigurationPropertySource source : sources) {
+			Assert.notNull(source, "Sources must not contain null elements");
+		}
 		this.sources = sources;
 		this.placeholdersResolver = (placeholdersResolver != null) ? placeholdersResolver : PlaceholdersResolver.NONE;
 		this.bindConverter = BindConverter.get(conversionServices, propertyEditorInitializer);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/BinderTests.java

@@ -70,19 +70,27 @@ class BinderTests {
 	private Binder binder = new Binder(this.sources);
 
 	@Test
-	void createWhenSourcesIsNullShouldThrowException() {
+	void createWhenSourcesIsNullArrayShouldThrowException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new Binder((ConfigurationPropertySource[]) null))
 				.withMessageContaining("Sources must not be null");
+	}
+
+	@Test
+	void creatWhenSourcesIsNullIterableShouldThrowException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new Binder((Iterable<ConfigurationPropertySource>) null))
 				.withMessageContaining("Sources must not be null");
 	}
 
 	@Test
-	void createWhenSourcesContainNullValuesShouldThrowException() {
+	void createWhenArraySourcesContainsNullElementShouldThrowException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new Binder(new ConfigurationPropertySource[] { null }))
-				.withMessageContaining("Sources cannot contain null values");
+				.withMessageContaining("Sources must not contain null elements");
+	}
+
+	@Test
+	void createWhenIterableSourcesContainsNullElementShouldThrowException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new Binder(Collections.singletonList(null)))
-				.withMessageContaining("Sources cannot contain null values");
+				.withMessageContaining("Sources must not contain null elements");
 	}
 
 	@Test