fixed test failures with respect to null handling and PropertyTypeDescriptor usage for collection elements

org.springframework.core/src/main/java/org/springframework/core/convert/support/GenericConversionService.java

@@ -172,7 +172,7 @@ public class GenericConversionService implements ConversionService, ConverterReg
 		}
 		GenericConverter converter = getConverter(sourceType, targetType);
 		if (converter == null) {
-			if (targetType.getType().isInstance(source)) {
+			if (source == null || targetType.getType().isInstance(source)) {
 				logger.debug("No converter found - returning assignable source object as-is");
 				return source;
 			}

org.springframework.core/src/main/java/org/springframework/core/convert/support/PropertyTypeDescriptor.java

@@ -117,4 +117,13 @@ public class PropertyTypeDescriptor extends TypeDescriptor {
 		return anns;
 	}
 
+	public TypeDescriptor forElementType(Class<?> elementType) {
+		if (elementType == null) {
+			return TypeDescriptor.UNKNOWN;
+		}
+		else {
+			return new PropertyTypeDescriptor(this.propertyDescriptor, getMethodParameter(), elementType);
+		}
+	}
+
 }