In Prepare>Data Reshape>Transpose Columns, ensured that New Names field is initially empty empty when Dialog is opened

[EstherNjeriLiberatta]

Fixes #7461
Removed NULL from the New Names receiver and cleaned up the code.
This is not ready for review.

[lloyddewit]

@EstherNjeriLiberatta Thanks for this.
Tip: if its not ready for review, it's best to click on Convert to draft.

[EstherNjeriLiberatta]

Thank you @lloyddewit

[EstherNjeriLiberatta]

I found these two bugs when fixing the dialogue:
The first image shows the selector not updating itself when I get back to the receiver Columns to Transpose. It should show all the columns of the dataset and not the columns previously selected.

The second image shows when the dialogue is reset. It is only showing the variables previously selected when both receivers are filled and not the columns of the whole dataset.

I have fixed these two bugs with help from @Ivanluv.
@rdstern @africanmathsinitiative/developers This is ready for review.