The previous code was calling the `mlflow.set_tracking_uri` function
regardless of whether or not the environment variable
`MLFLOW_TRACKING_URI` is even set. This led to clashes with the original
MLflow implementation and therefore the logic was changed to only
calling the function when the environment variable is explicitly set.

The previous code did not consider the possibility that the tracking URI
may already be set elsewhere and was therefore (erroneously) overriding
previously set tracking URIs using the environment variable.

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

"Unset by default" is the correct expression rather than "Default to `None`."

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>