Handle null exception of encrypt()

[tristaZero]

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Which version of ShardingSphere did you use?

4.0.0-RC1-SNAPSHOT

Which project did you use? Sharding-JDBC or Sharding-Proxy?

shardinng-jdbc

Expected behavior

Actual behavior

When query the data that is null or "" in datasource, sharding-jdbc will throw null excpetion.

Reason analyze (If you can)

Steps to reproduce the behavior, such as: SQL to execute, sharding rule configuration, when exception occur etc.

Example codes for reproduce this issue (such as a github link).