[SPARK-13697] [PYSPARK] Fix the missing module name of TransformFunctionSerializer.loads

## What changes were proposed in this pull request?

Set the function's module name to `__main__` if it's missing in `TransformFunctionSerializer.loads`.

## How was this patch tested?

Manually test in the shell.

Before this patch:
```
>>> from pyspark.streaming import StreamingContext
>>> from pyspark.streaming.util import TransformFunction
>>> ssc = StreamingContext(sc, 1)
>>> func = TransformFunction(sc, lambda x: x, sc.serializer)
>>> func.rdd_wrapper(lambda x: x)
TransformFunction(<function <lambda> at 0x106ac8b18>)
>>> bytes = bytearray(ssc._transformerSerializer.serializer.dumps((func.func, func.rdd_wrap_func, func.deserializers)))
>>> func2 = ssc._transformerSerializer.loads(bytes)
>>> print(func2.func.__module__)
None
>>> print(func2.rdd_wrap_func.__module__)
None
>>>
```
After this patch:
```
>>> from pyspark.streaming import StreamingContext
>>> from pyspark.streaming.util import TransformFunction
>>> ssc = StreamingContext(sc, 1)
>>> func = TransformFunction(sc, lambda x: x, sc.serializer)
>>> func.rdd_wrapper(lambda x: x)
TransformFunction(<function <lambda> at 0x108bf1b90>)
>>> bytes = bytearray(ssc._transformerSerializer.serializer.dumps((func.func, func.rdd_wrap_func, func.deserializers)))
>>> func2 = ssc._transformerSerializer.loads(bytes)
>>> print(func2.func.__module__)
__main__
>>> print(func2.rdd_wrap_func.__module__)
__main__
>>>
```

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11535 from zsxwing/loads-module.