[SPARK-19163][PYTHON][SQL] Delay _judf initialization to the __call__

## 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.