Bind empty string to empty collection or array of rich types

Previously, when an empty String was bound to a collection or
array of rich types it would fail as there was no converter capable
of creating a Collection<RichType> or RichType[] from the String.

This commit updates IndexedElementsBinder to apply special treatment
to empty String values. Now, when such a value is being processed,
an empty Collection or array is the result.

Closes gh-12965

Author: wilkinsona

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

@@ -18,6 +18,7 @@
 
 import java.lang.annotation.Annotation;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.TreeSet;
 import java.util.function.Supplier;
@@ -32,6 +33,7 @@
 import org.springframework.core.ResolvableType;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.StringUtils;
 
 /**
  * Base class for {@link AggregateBinder AggregateBinders} that read a sequential run of
@@ -81,11 +83,17 @@ private void bindIndexed(ConfigurationPropertySource source,
 			ResolvableType aggregateType, ResolvableType elementType) {
 		ConfigurationProperty property = source.getConfigurationProperty(root);
 		if (property != null) {
-			Object aggregate = convert(property.getValue(), aggregateType,
-					target.getAnnotations());
-			ResolvableType collectionType = ResolvableType
-					.forClassWithGenerics(collection.get().getClass(), elementType);
-			Collection<Object> elements = convert(aggregate, collectionType);
+			Collection<Object> elements;
+			Object value = property.getValue();
+			if (value instanceof String && !StringUtils.hasText((String) value)) {
+				elements = Collections.emptyList();
+			}
+			else {
+				Object aggregate = convert(value, aggregateType, target.getAnnotations());
+				ResolvableType collectionType = ResolvableType
+						.forClassWithGenerics(collection.get().getClass(), elementType);
+				elements = convert(aggregate, collectionType);
+			}
 			collection.get().addAll(elements);
 		}
 		else {

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

@@ -363,6 +363,19 @@ public void bindToBeanWithNestedCollectionShouldPopulateCollection() {
 		assertThat(foo.getFoos().get(1).getValue()).isEqualTo("three");
 	}
 
+	@Test
+	public void bindToNestedCollectionWhenEmptyStringShouldReturnEmptyCollection() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo.value", "one");
+		source.put("foo.foos", "");
+		this.sources.add(source);
+		Bindable<BeanWithNestedCollection> target = Bindable
+				.of(BeanWithNestedCollection.class);
+		BeanWithNestedCollection foo = this.binder.bind("foo", target).get();
+		assertThat(foo.getValue()).isEqualTo("one");
+		assertThat(foo.getFoos()).isEmpty();
+	}
+
 	@Test
 	public void bindToCollectionShouldUsePropertyEditor() {
 		// gh-12166