## What changes were proposed in this pull request?
Defer `UserDefinedFunction._judf` initialization to the first call. This prevents unintended `SparkSession` initialization. This allows users to define and import UDF without creating a context / session as a side effect.
[SPARK-19163](https://issues.apache.org/jira/browse/SPARK-19163)
## How was this patch tested?
Unit tests.
Author: zero323 <zero323@users.noreply.github.com>
Closes#16536 from zero323/SPARK-19163.