[ROCKETMQ-351] Add a default value to expressionType and polish the j…

[vsair]

What is the purpose of the change

Make the expressionType has a definite value.

Brief changelog

1. Add a default value to expressionType of SubscriptionData.
2. Empty check in isTagType of ExpressionType.

Verifying this change

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[vsair]

@lizhanhui @zhouxinyu @lindzh pls have a review.

[coveralls]

Coverage decreased (-0.05%) to 39.905% when pulling 5d4dac1 on vsair:ROCKETMQ-351 into cc0b3d0 on apache:develop.

[zhouxinyu]

LGTM~

[lizhanhui]

Looks good.

[vongosling]

+1