Add option for allowing null value on foreach tag

[kazuki43zoo]

I will propose to support new option for allowing null value on collection attribute of foreach tag.

In this change, a developer can omit the if tag as follow:

• current version

<if test="friendList != null"> <!-- Need null check using 'if' tag -->
  <foreach item="item" collection="friendList" open="id in (" separator="," close=")">
    #{item.id}
  </foreach>
</if>

• new version

<!-- <if test="friendList != null"> -->
<!-- Specify 'nullable="true"' instead of 'if' tag -->
<foreach item="item" collection="friendList" nullable="true" open="id in (" separator="," close=")">
  #{item.id}
</foreach>
<!-- </if> -->

Also, a developer can change the default nullable value when the nullable attribute is omitted using global configuration as follow:

• global configuration

XML configuration (mybatis-config.xml):

<settings>
  <setting name="nullableOnForEach" value="true"/>
</settings>

or

Java configuration(Configuration class):

Configuration mybatisConfiguration = new Configuration();
mybatisConfiguration.setNullableOnForEach(true);

• SQL definition

<!-- <if test="friendList != null"> -->
<!-- Can omit 'if' tag and 'nullable' attribute both -->
<foreach item="item" collection="friendList" open="id in (" separator="," close=")">
  #{item.id}
</foreach>
<!-- </if> -->

WDYT?

[kazuki43zoo]

@mybatis/committers WDYT this PR?

[harawata]

Thank you for your work, @kazuki43zoo !

This gets a few upvotes and the impact looks small, so I'm OK with it.

[kazuki43zoo]

@harawata Thx for your reaction!! I will rebase this PR.

[coveralls]

Coverage increased (+0.02%) to 87.271% when pulling 7738026 on kazuki43zoo:allow-null-on-forEach into 6490a16 on mybatis:master.

[kazuki43zoo]

@harawata I've rebased and update doc.

[harawata]

Thank you, @kazuki43zoo !
LGTM 👍