[SPARK-16231][PYSPARK][ML][EXAMPLES] dataframe_example.py fails to convert ML style vectors
[SPARK-16231][PYSPARK][ML][EXAMPLES] dataframe_example.py fails to convert ML style vectors

## What changes were proposed in this pull request?
Need to convert ML Vectors to the old MLlib style before doing Statistics.colStats operations on the DataFrame

## How was this patch tested?
Ran example, local tests

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #13928 from BryanCutler/pyspark-ml-example-vector-conv-SPARK-16231.

(cherry picked from commit 1aa191e58e905f470f73663fc1c35f36e05e929a)
Signed-off-by: Xiangrui Meng <9616cf314f60cbadfd3d5d51110130ad7923defb@databricks.com>

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