## What changes were proposed in this pull request?

Improve PrefixSpan pre-processing efficency by preventing sequences of zero in the cleaned database.
The efficiency gain is reflected in the following graph : https://postimg.org/image/9x6ireuvn/

## How was this patch tested?

Using MLlib's PrefixSpan existing tests and tests of my own on the 8 datasets shown in the graph. All
result obtained were stricly the same as the original implementation (without this change).
dev/run-tests was also runned, no error were found.

Author : Cyril de Vogelaere <cyril.devogelaeregmail.com>

Author: Syrux <pokcyril@hotmail.com>

Closes #17575 from Syrux/SPARK-20265.