Fix NullPoint exception caused by missing TypeHandler

Nested results were handled differently, a resultMap can run in simple query but not in netsted map, e.g.
```xml

    <resultMap id="referOrdersMap" type="java.lang.String">
      <result property="value" column="ref_id"/>
    </resultMap>

    <resultMap id="productResult" type="org.apache.ibatis.submitted.nestedresulthandler.Product">
		<result property="id" column="product_id" />
        <!-- use resultMap="referOrdersMap" instead of column="ref_id" to verify NPE in NestResultHandler-->
		<collection property="referOrders"  resultMap="referOrdersMap" javaType="list" ofType="java.lang.String"/> 
	</resultMap>

     <select id="getProducts" resultMap="productResult">
        select 'ID:'||order_id ref_id ,product_id
        from orderDetail
        order by product_id
     </select>

    <select id="getSimpleResult" resultMap="referOrdersMap">
        select 'ID:'||order_id ref_id from orderDetail
    </select>
```

getSimpleResult can run successfully but getProducts will throw null point exception.

The fix added null checking in createRowKeyForMappedProperties method and fall back to resultColumnCache.getTypeHandler if no type handler found.