Allow auto grow for entries w/o default constructor in SpEL

Prior to this commit, The "auto grow" feature in SpEL expressions only
worked for element types with a default constructor. For example, auto
grow did not work for a list of BigDecimal elements.

This commit inserts a null value in the list when no default
constructor can be found for the element type.

Closes gh-25367

spring-expression/src/main/java/org/springframework/expression/spel/ast/Indexer.java

@@ -712,10 +712,10 @@ public class Indexer extends SpelNodeImpl {
 				}
 				TypeDescriptor elementType = this.collectionEntryDescriptor.getElementTypeDescriptor();
 				try {
-					Constructor<?> ctor = ReflectionUtils.accessibleConstructor(elementType.getType());
+					Constructor<?> ctor = getConstructor(elementType.getType());
 					int newElements = this.index - this.collection.size();
 					while (newElements >= 0) {
-						this.collection.add(ctor.newInstance());
+						this.collection.add(ctor == null ? null : ctor.newInstance());
 						newElements--;
 					}
 				}
@@ -725,6 +725,15 @@ public class Indexer extends SpelNodeImpl {
 			}
 		}
 
+		Constructor<?> getConstructor(Class<?> type) {
+			try {
+				return ReflectionUtils.accessibleConstructor(type);
+			}
+			catch (Throwable ex) {
+				return null;
+			}
+		}
+
 		@Override
 		public boolean isWritable() {
 			return true;