Update pyspark.py to include function for carrying out left joins where the dataset contains many nulls.

[schofr2]

Description

This change is a new feature. it is a function for carrying out left joins where there are many nulls in the left hand side dataset.
It has not been extensively tested - this will need to be done if incorporated - I'm happy to change this pull request to merge into a feature branch on your side if you wish to develop this further.

Checklist:

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation (docstrings)

Peer review

no peer review has been done.

[dombean]

Thanks for the contribution! A few requests to align with project standards:

• Thanks for adding type hints to the function signature and docstring.
  • Could you please remove type hints from the Parameters section of the docstring to reduce duplication -- they’re already present in the signature. See: Function House Style
• Could you add unit tests following our house style? Guide here: Unit Test House Style
• Please add an Examples section to the docstring to illustrate usage.