MultiSlideTextUpdate empty list causes corrupted output files

#### Description:

When pushing a `MultiLineTextUpdate` with an empty list, the output presentation becomes corrupted.

#### Steps to reproduce:

Push a MultiLineTextUpdate with an empty list.

#### Expected behaviour:

Either:
* The text box should not be updated

or
* The text box should be updated to be empty.

#### Test file(s):

Start by locating the MultiLineTextUpdate handling in the PowerPoint_Toolkit and reproduce the issue with an empty list. Determine whether the intended behavior is to leave the text box unchanged or make it empty, then verify that the resulting presentation is no longer corrupted.

Written by the indexing model from the issue text.