TypeDescriptor's nested type traversal leniently returns null in case of unresolvable nested type

Issue: SPR-11898
(cherry picked from commit d663580)

spring-core/src/main/java/org/springframework/core/convert/TypeDescriptor.java

@@ -701,8 +701,7 @@ public class TypeDescriptor implements Serializable {
 			}
 		}
 		if (nested == ResolvableType.NONE) {
-			throw new IllegalStateException(
-					"Unable to obtain nested generic from " + typeDescriptor + " at level " + nestingLevel);
+			return null;
 		}
 		return getRelatedIfResolvable(typeDescriptor, nested);
 	}